when 'fewer' and 'less' and 'under' are synonymized?

What the Merriam-Webster Dictionary of English Usage has to say about less, fewer (link). This rule was invented in 1770 by Robert Baker in his Reflections on the English Language (link).
Quote:
XLVII. LESS.

THIS Word is most commonly used in speaking of a Number; where I should think Fewer would do better. No fewer than a Hundred appears to me not only more elegant than No less than a Hundred, but more strictly proper.
As the MWDEU entry goes on to show, the earliest examples of less being used with countables goes back, at least, to King Alfred in his Latin to Old English translation in 888 CE. Baker was another of those cranky 18th century linguists who hoisted their opinions on usage in rules of correctness.

Another interesting thing about less is that it is the suppletive comparative form of little. The form lesser is a double comparative akin to betterer.

Ceci n'est pas un seing.