how are they different?

i dont know if my mates golf is the same, but i know it has lots of unused crew threads so look like u could bolt everything on easily enough
 
Just realised you mean the rocker cover and not the crappy plastic engine cover. In that case yes most are the same, however some have slight differences depending on where bits bolt up onto.

Do a search and you'll see the rocker cover topics have been covered a few times recently and differences have been highlighted.
 
I did a search just for "cam cover" and "rocker cover"

Neither brought up recent topics regarding differences in leon and ibiza etc?