imskpwr wrote:Even if this difference is very subtle, I still want to know as it was given in an official explanation.

unfortunately, a small percentage of the og explanations are actually wrong. for instance, the og explanation of #77 identifies a certain structure as parallel when, in fact, it's not a parallel structure at all (the fact that both parts are infinitives is just a coincidence).
looking at the official guide, you can definitely tell that GMAC employs its less able writers to create the answer explanations, while its best people are clearly writing the questions (which almost never have any issues). this situation is clearly preferable to the reverse -- it would be horrible if the questions themselves have these problems -- but it's still unfortunate.

here's a GMAT PREP problem in which the correct answer uses "since" to mean "because":
https://www.beatthegmat.com/gold-t10765.html

also, i want to give a strong endorsement to what jim is saying above -- do not try to learn everything in SC. as the old saying goes, if you try for everything, you'll get nothing.

in fact, if you can master just the following five subjects --
meaning of the sentence
parallelism
pronouns
subject-verb agreement
modifiers
-- then you will already be able to solve something like 80-90 percent of all the problems in SC. that's a high enough percentage to earn you whatever score you might want; the other 10-20 percent really doesn't matter very much.