That's an interesting Q. My first guess is the same as yours, you're getting a little forward and maybe hips shooting up a bit before it "bites," and when it does, you've already extended your knees so you're mostly relying on hammies and low back at that point.
If your form is spot on, however, I wonder if the higher hip height with the heel on might have something to do with it. Yes, the knee angle is more closed and thus hammie less under strain from that end, but from the proximal end the hips are higher and that lengthens the hammy out again. I haven't measured anything so not sure about this exact interplay, but maybe the raising of hips lengthens the hammies more than the slightly more forward/flexed knees shortens them - or maybe it does so in people with certain anthropometries specifically - leading to initiating the pull from position where the hamstring is more engaged. Just an initial guess.