It's not all that uncommon for that to happen. When I got my 4 pairs of GBR's in the mail, two pairs spawned in under a week. Another spawn went by for each of the four pairs and then a week later 3 pairs spawned at the same time and the fourth was an hour later. That was pretty cool to see. I think it is the change in water temp that makes them do this when they come in the mail like that.
I noticed in the pic that there were few eggs laid close together which means you probably took a pic after most of them were eaten. Usually they lay a decent amount of eggs in a really small amount of space. I took a pic once and opened it with MS paint and erased the eggs one by one for a count... I got over 450 of them. They will most likely lay eggs for you every week or so, so you may want to prepare for that. I am about to take a spawn out of my 55 gallon tank and try to hatch them in my 10. Even bought some brine shrimp eggs in hopes of successfully raising them longer than the 5 days past free swimming as is my best so far. Congratulations BTW.