Basically it's a matter of what's easier to say - "Jesus's" is more awkward than "Jesus'" "Truss's" is easier than "Truss'". At least to me it does.

Fowler is clear that when there is an extra s it should be pronounced, and that when it isn't there it is shouldn't be pronounced. The possessive apostrophe would always be there (unless one was following George Bernard Shaw, and leaving out all apostrophes everywhere, as a matter of principle). The puzzle was as to when the apostrophe should be followed be an additional s, and on what occasions it might be correct to dispense with that additional s.

It was formerly customary, when a word ended in -s, to write its possessive with an apostrophe, but no additional s, e.g. Mars' Hill, Venus' Bath, Achilles thews. In verse, & in poetic or reverential contexts, this custom is retained, & the number of syllables is the same as in the subjective case, e.g. Achilles' has three not four; Jesus or of Jesus, not Jesus's But elsewhere we now add the s & the syllable, Charles's Wain, St James's not St James', Jones's children, the Rev Septimus's surplice, Pythagoras's doctrines.(Fowler 1926)

(I see that the spell checker built into Google, though recognising at least some English English spellings, is insistent that Fowler here is wrong and that the "formerly customary" system is still current. Presumably this indicates that in America it is.)