Day 5 vs Day 6?

Just finished our sixth round of IVF (woof) and we ended up with two day five blasts (4AA and 3BB) and one day six blast (4AB). Our successful FET of a day six 4BB is our lovely 2.5 year old, so we've had luck with a day six before. My question is, I assume that blasts are checked at the same cadence every day. Is the difference between a day five and a day six potentially morning versus afternoon of the same day or is it more meaningful than that from a cellular perspective? I've read that speed of progression is more important than grading and that day five/day six blasts are pretty close in pregnancy and live birth stats. We did have two failed day seven FETs late last year and earlier this year that our RE wasn't too concerned about given lower day seven success rates (though they were both 6AA). Just looking for any technical input, thank you!