question about blastocyst development

[Anonymous]

So I'm one of those fantastically unlucky people who has had no blasts after 2 rounds of IVF. I requested my records to consult elsewhere and found something that bothered me, that nobody had mentioned to me previously.

I had one embryo that was rated "EACAV" on day 6. That means, early cavitating... as in almost a blastocyst, right?? I'm pretty sure this embryo came from one of my late maturing eggs.

They have the progression go like this
Day 2: 2 cell
Day 3: 6 cell
Day 5: no change
Day 6: EACAV

So my question is... How does it go from no change in 2 days to almost a blastocyst overnight?? Did the lab screw up? Or is this just something that happens?

(I realize it's a bad sign to not be a full blastocyst by day 6, but given this embryo had a late start, I'm kinda upset this embryo was discarded and not given a chance so I'm dwelling on this)