My partner had a vasectomy about 1.5-2 years ago now.

He recently just went and had a sperm sample taken and the preliminary came back positive for sperm 3 different times.

They sent it off for the actual analysis and the analysis is showing negative, zero sperm at all. What would be the reason for this? He has an urology appointment set up but I’m confused on why his preliminary would come back positive so many times but then nothing on the analysis? TIA.