The maximum shutter speeds reported by the D750 in A mode are:
60p: 1/100
50p: 1/50
25p: 1/25
24p: 1/25
Anyway, A mode does have a possible advantage. The camera likely chooses real multiples of frame-rates for shutter speeds, regardless of what it displays as being used (=roughly rounded values). In M mode it's well possible that it either uses exactly the value you set up (usually not real multiples) or still keeps using multiples internally while displaying rounded values. We still don't know, but changes are a bit higher that A modes does it right in any case. How much that matters in practice is an entirely different matter.
Good that we have choice between M and A at least, even when I don't understand why A is not allowed to use manual ISO.