Does 差不多 (chàbùduō) imply "slightly less than" when comparing quantities?
差 = 相差 (difference) ;差距 (gap)
不多 = not much

差不多 :
(adj) not much difference/ not much gap between
(adv): almost / about

Example of 差不多 as adjective:
味道差不多 (not much difference in taste) - could be slightly better or slightly worse
價格差不多 (not much difference in price) - could be slightly higher or slightly lower
Example of 差不多 as adverb:
差不多到了 means &quot;we're almost there&quot; (not quite there yet, but close to)
差不多死了  means &quot;almost died&quot; or &quot;about to die&quot; ( not dead yet but will be soon)
When A ask B how much is the book,  and B said: &quot;差不多50块&quot;, it mean &quot;it is almost 50 yuan&quot; (= not quite 50 yuan)
When A found out the actual price is 52 yuan, he has every right to accuse B of lying.
And B's excuse: &quot;52块, 50块 差不多吧?&quot; (52 yuan and  50 yuan is not much a difference right?) is using 差不多 in a different way.

Does 差不多 (chàbùduō) imply “slightly less than” when comparing quantities?

When 差不多 is used  as an adverb (is almost) + quantities, it dose imply “slightly less than”, But when 差不多 is used as an adjective (not much difference), then it only mean the objects are similar in some form of comparison.

A 比 B 差不多

The correct form:
A 跟 B 差不多 = not much difference between A and B
A 比 B 差不了多少 = Compare to B, A is not much worse
A 比 B 差不多 is not grammatical