The usual causes, roughly in order:

1. Heat. Laptops especially will slow themselves down after sustained load. A second run often lands lower than the first purely for this reason.
2. Other software. A browser with many tabs, or anything using the GPU, competes for exactly the resource that limits generation speed.
3. Cold vs warm model. The first run after loading includes work that later runs skip.

This is why comparisons should be run alternating — A, B, A, B — rather than all of A then all of B. Otherwise you're partly measuring how warm the machine got.

If your numbers swing by more than about 20% with nothing else running, that's worth posting about.