Problem is, conversion rate is such a woolly statistic, and everyone has their own definition of what a shot is and lots of other auxiliary junk. Opta stats put RVP's conversion rate - which they have a dedicated stat for - at 22%, compared to Whoscored putting him at about 18%.
Oh I agree, different sites use different methods, but I'll hazard a guess and say that Aspas' numbers are still below average using Opta's numbers.
That said, it's a bit harsh on Aspas to judge him on stats alone, they say stats don't lie but when you play the lone striker role for a relegation-battling side like Celta you don't always get the service you need and sometimes you're forced to take tough shots -which will count against you- because there really isn't much more you can do. Furthermore, if we are to judge Aspas on stats alone, then we shouldn't forget that he was not only Celta's top scorer, but he also lead them in assists and key passes, which highlights just how much Celta relied on him for pretty much everything.
But that has nothing to do with Chelsea, though.