Well, if Dembele is 120+30, that means that we will probably pay 120m now and 30 in the next 2-4 years, based on variables.
Now, if you count only 120m for Dembele into this year and 30m into upcoming years, then you have to add addons from previous years into a current year.
For example, we are probably currently paying addons for Umtiti, Mats, Rakitic, Suarez, Denis, Gomes, Paco etc
Majority of them had some addons in terms of trophies, games played, goals scored etc
So, you can't say: it is 300m now and 120m in the future.
Fine, but then you have 300m now, 120 in the future, and probably 50-ish m in addons this summer from previous transfers.
So, it evens out to some extent and it is again way higher than only 300 for fixed amounts from this summer.