Did the same labs run both tests? I had 3 SPEPs done in a 2 month period. Two of them were sent out to national labs and came back 1.5. The other one was done at a national cancer center and was 1.32. I don't think my numbers changed at all, I think one test was just more accurate than the others.

You didn't mention whether this was a random drop or a drop after treatment. A random drop is obviously more suspicious.

Also, if you have trouble with issues like hydration or kidney issues those can cause fluctuation in some lab values. My sister is an engineer and always trying to define precision and accuracy to tell me why my numbers aren't changing. (I took stats too, girl.) I don't agree with everything she says, but if something looks like a big change I think it's worth retesting just to confirm that it's a real change.