What's the difference between reckon and underreckon?

Reckon

Definition:

(v. t.) To count; to enumerate; to number; also, to compute; to calculate.
(v. t.) To count as in a number, rank, or series; to estimate by rank or quality; to place by estimation; to account; to esteem; to repute.
(v. t.) To charge, attribute, or adjudge to one, as having a certain quality or value.
(v. t.) To conclude, as by an enumeration and balancing of chances; hence, to think; to suppose; -- followed by an objective clause; as, I reckon he won't try that again.
(v. i.) To make an enumeration or computation; to engage in numbering or computing.
(v. i.) To come to an accounting; to make up accounts; to settle; to examine and strike the balance of debt and credit; to adjust relations of desert or penalty.

Example Sentences:

(1) And, according to a letter leaked to the BBC last week , he reckons he has found one: default-on.
(2) The two companies have pooled their software development resources to create MeeGo, a free software platform which they reckon will pave the way for the next generation of wireless communications devices.
(3) Chelsea might recover under similar circumstances, but I reckon they need a pretty big overhaul.
(4) When I joined, Francis said, I reckon we've got three or four more years left."
(5) 12.37pm BST Genworth , which sells mortgage insurance in the UK, also reckons any impact from today's measures will mainly fall on London.
(6) And none of them are making money, they are all buying revenue with huge war chests.” Patrick reckoned the 2.0 tech bubble will come to be defined by the unicorn.
(7) Even so, Byrne reckons that they will move to an embedded version of Windows 7 for ATMs over the next 18 months or so.
(8) An array of polling proves that the 50p rate is unanswerably popular: at the time it was introduced, Populus reckoned that 57% of people were in favour, as against only 22% against; and a subsequent poll by YouGov found that keeping the 50p rate would appeal to 88% of uncommitted voters.
(9) Carney will have to defend his bold pledge to peg UK interest rates to their current record low of 7% until unemployment rate has dropped to 7%, sometime in 2016 by the Bank's reckoning.
(10) While this is something that gives substance to the familiar cry of “Never again,” it will be up to the countries in the western Balkans, and in particular Bosnia and Herzegovina, to engage in an honest reckoning with the past, rather than narratives based on chauvinism or denial.
(11) Elsewhere in Tripoli, a Ghanaian reckons some of his friends would have stayed in Libya if the country was stable.
(12) Despite the "immense challenges" which Yves Mersch cited today , BNP reckons the ECB will have to take unconventional action to fight off weak inflation and to stimulate growth.
(13) Another possible way to minimize the effects of "noise" is to increase the size of the samples on which the reckon ing is based.
(14) Elisabeth Afseth, bond market expert at Evolution Securities, reckons that the first pointer of a fresh credit crunch was returning could be seen on August 18 this year when the European Central Bank revealed that one bank had borrowed $500m for a week – as it could not find the money on the open market.
(15) Albeit an unloveable, slightly scary Ron Burgundy in a 'I may now be a low level Tesco manager in a cheap suit but I still remember how to handle a stanley knife' kind of way," reckons Robert Lowery, who is forgetting that Jim White has a phone.
(16) "If my math is correct, if Costa Rica score a second, Uruguay will only need a draw to progress alongside Los Ticos," reckons Vitor Ta.
(17) Children are their parents’ biggest investment: the cost of a child from birth to graduation is now reckoned to be £227,000 (Centre for Economic and Business Research, 2014).
(18) Since 2004, he reckons, the lab has spent around £6m on research in total, about half raised from European grants and the rest from projects with South Korean and American corporations.
(19) Simultaneously it is interesting to reckon the new aspects which are raised with the evolution of these methodologies such as the responsibility of decisions taken by intelligent systems, the probable advantages, at the present stage, of the interactive systems and the risk of self-learning systems.
(20) It's very reminiscent of a similar death almost a year ago, when a "middle-aged trade unionist" collapsed and died during a protest ( details ) Updated at 1.42pm BST 1.31pm BST 30,000 join Athens protests Reuters reckons that more than 30,000 people took part in today's demonstrations in Athens, and that the trouble began when "a small group of protesters" began throwing marble, bottles and petrol bombs at the ropt police who were "barricading part of the square".