They do occasionally have a blip just as humans do - but I think a gap of 9 months with nothing in between seems a bit long. Are you sure she didn't have a mini one somewhere in the middle? My younger girl (now
did that a few months ago so when she came in again I was taken by surprise as I was then too late to have her spayed - that had been the intention, and it needs to be done half way between seasons.
Perhaps a word with your vet just to reassure you would be a good idea - a phone call might be enough.