It's certainly not an exact science. It is believed that there are things that can elevate your PSA temporarily as well, certain activities, being dehydrated, cycling etc.. I'm certainly no expert but that does seem like quite a drop.

Also, I'm a little reluctant to mention but there have been cases of false negatives in the biopsies. I believe I've read here that there was even that was positive, with low Gleason numbers, then a follow up a few months later was negative.

If you're referring to a trans rectal with no prior MRI having positive results for guidance, depending on how many cores/samples are taken, it's possible to take the samples from areas with no cancer. Hopefully that is not the case here.

If it were me, I'd request regular blood tests till you know it's fully stabilized for peace of mind. Stay positive but vigilant. Here's hoping it's just a blip in your PSA numbers!