What is the difference between Future Perfect Indicative and Perfect Subjunctive

Question

In Reginaldus Foster's text book Ossa Latinitatis Sola, he states that, apart from the First Person Singular, the forms for Future Perfect Indicative and Perfect Subjunctive are the same. And, indeed, looking at the paradigms in John C. Traupman's New College Dictionary and W. Michael Wilson's Essentials of Latin Grammar confirms that this is so.

However, when I look at the paradigms in my First Year Latin books, namely Wheelock's 7th edition, and at Gavin Betts' Teach Yourself Latin, I see some slight differences. For example, there I find under amo amāverimus for the Future Perfect and amāverīmus for the subjunctive. (If you missed it, the i is long in the subjunctive, but not in the indicative.) The length of the i in some of the other forms change as well.

My question is, what is the cause or back story behind this disagreement?

Answer 1

Alex B. quotes from Weiss 2009/2011 page 420:

Although etymologically the stem vowel of the perfect subjunctive should be ī and the stem vowel of the future perfect should be ĭ, Classical Latin poets use both long and short -i- indiscriminately in the 2nd [singular], 1st [plural], and 2nd [plural] of both paradigms.

Some paedagogical works (Henle, Wheelock) say the vowel lengths in these three forms are are different; others (Ørberg) say they're all short. But this seems to be a later invention based on the etymology, instead of on actual Classical-era usage.