Is this normal?

We had retrieved 7 eggs; 6 were fertilized and all 6 made it to Day 5. We transferred two embryos yesterday (very exciting) but our doctor called us today to let us know that the remaining 4 didn't make it. We now don't have any to freeze which is a little disappointing but even though they told us that the two yesterday were of high quality, how can that be? Given that at least two others were as well but today they didn't make it.