What's the mechanism for that? Maybe a few sperm are stuck in the sperm ducts past the point they were severed, but the ducts are cut and sealed, aren't they? Even if they weren't sealed, it would be Russian Roulette trying to get across the gap in what's effectively just body cavity space.

My expectation would be the first sample might have lower but significant sperm count, and each subsequent sample would decrease dramatically until any residual sperm died and the count was zero. You're talking about motility, not count, but it doesn't sound like you noticed a drastic decrease from sample to sample. That doesn't seem right according to my understanding.