My car’s a 2020 Polo GTI Plus which has the same type of key fobs as the mk7 / mk7.5 Golf GTI.
When I bought my car, the key fobs came with both the GTI chrome and the black plastic end caps (two of each) so I could choose whether to make them GTI-specific or generic VW. I’ve got the black plastic end cap on the key fob I use most often as the plastic cap is a little smaller than the GTI chrome one and it fits comfortably in my silicone key fob case. With the chrome cap fitted, the fob doesn’t fit so well in the silicone key case.
@KieranD79; maybe the previous owner also used a silicone key fob case, hence why one of the key fobs for the car you’re buying has the black plastic end cap fitted.