Where are you getting marketing from the net spend?
To put it simply FFP is where you cannot outspend what you make.
United is the biggest club in the world marketing wise meaning that it's got one of the highest revenues in the world, and has access to more funds. All the players we've bought under the Glazers has been money coming from the club's revenue and the Glazers never spent one penny of their own money on the club and just like to buy shirt-selling players.
Compared to City... What they do (one of the 115 things they're being charged for) is they get "sponsors" like "Mansour Enterprises", which is literally just a fake business where state money can get pumped into it, allowing them to make it look like they can buy all of their players without breaking any rules (they cannot). Newcastle are doing the same which is why they binned off their sponsors (sports direct, etc..).
Our net spend is higher due to terrorists like the Glazers and Ed Woodward whilst City's entire infrastructure that has allowed for a decent net spend & high investments is based on state-owned money and not money the club made from being a football club.
This was always the case because how could a team that loses 8-1 to Middlesbrough & battle with relegation the next season be able to actually buy Robinho. It's only being charged now as there's been a 10+ year investigation on it.
It's easier to run a football club as a football club when you're running it for political leverage (state-owned) vs. for profits (Glazers).