Year to year spend is misleading if you've spent 500m the year before. Just because another team outspends you in a year where you don't need to outspend the Vatican doesn't erase what has already been done.
Pep has been a very successful manager, yeah, but ' he's ' also spent a ridiculous amount of money.
Also, for what it's worth, I think 'net spend' is relevant also since generally to have a low net spend you'll have to sell good players for good money (Coutinho for example, even though everyone knows how happy I was about that) and reinvest wisely. Very different from just buying amazing players over and over without ever needing to sell. Just as an example Pep has a net spend of 400m pounds with City (Liverpool with a meager 72m).