Fin performance does not seem to alter over time, but rather due to condition (warped fin, jagged rear edge can reduce performance). More likely the Drives are set up and/or lubed differently. Drive friction varies greatly with cable tension and lube. This becomes apparent if you operate the Drives (by hand) out of the water.
There are three generations of fin material, each with a slightly different feel. I assume your 15 month old fins are the latest. There is no marking to identify the difference.
Of course, how the fins are mounted (variations in the outhaul clew setting) noticeably affects both performance and resistance.
Just guessing, but I suspect different Drives using the same fins can vary up to about 30% in performance depending on how everything is set-up and maintained.