I think you have to look at a club's overall money available every year to spend, if we profited say 180M every year, but we had to use 80M of that to pay back debt, then there is 100M left to spend, thats what you can spend or try to balance to after a transfer window. You can afford to have a net spend of 100M. Now if Arsenal for example profited 70M every year, but they didn't have to pay any debts back, then they could afford to have a net spend of 70M and be fine.
Technically our net spend is 30M higher, but in the context of all finances and profits and shit (and I haven't even mentioned the potential extra money signing new players can bring in from marketing stuff), our net spend is effectively the same. We've spent within our means in the exact same way. Accumulate that over 8 windows and our net spend would be 240M higher in this theoretical situation despite spending the same as them relative to our overall profits (not just transfer sales).
Having more money and spending more money, you also often have to fork out more for players. Players are talked about as 50M pound players when we're linked to them but then a while later you hear about them going to another bundesliga club or serie A club for example for like 20M. The clubs with big revenues also tend to pay bigger wages to players, and so there is a tendency to get a fair bit less back when we sell players than we should. This obviously has an effect on net spend too.
As others have mentioned holding on to players and keeping them until they leave for practically nothing or retire at United means basically no resale value, we've done that with a decent number of players in the past 8 years.
I'd be interested to see a split for this list of the biggest spending clubs of the last 8 years. 1 for the last 5 years and 1 for the 4 years before that. I don't we'd figure that highly at all on a spending ranking for spending 2010-2013. Net spend would be interesting to see here too.