fov_desired (TF2/src's world FoV option) doesn't directly translate to your in-game horizontal FoV. I mean, it does for 4:3, but who plays on that aspect ratio anymore? Just joking. Now, your FoV is capped at 90. But if you use an aspect ratio higher than 4:3, it's wider than what fov_desired is set to, or what you put in the options. As it is right now, tf2 effectively uses a ~vertical~ FoV at 3/4 what's selected. This makes the max horizontal FoV at different aspect ratios different. In a perfect world, everyone would use the same aspect ratio (I vote for 16:10) but we don't have that. As it is, people with 4:3 monitors are at a tremendous disadvantage to people with 16:9 monitors, regardless of what they do in their options. There's no perfect solution to this. If we switch it around to a capped horizontal FoV, people with 4:3 monitors would have an advantage over people with 16:9 monitors. Yes, the vertical field of view is actually a pretty big deal in a lot of cases. Anyways, right now, the normal max FoVs are as follows:
4:3   - 90
16:10 - 108
16:9  - 120
I find it funny when people complain about having a capped FoV of 90, and talk about how good ol' quake 3 and quake live let them set it to whatever they want, and ask for the FoV cap to be raised to 120, when in reality, they already have access to that. Unless they have a 4:3 monitor. But you can't solve that without making the vertical FoV dynamic, which makes vertical gameplay - one of TF2's hardest to master and most impressive aspects - the part of it that can be unfair. One possible "solution" to the dillemma is to make it so that you cap the combined horizontal and vertical FoV, so that 4:3 players aren't at a direct downgrade to 16:9 players, but then people will just practice one aspect of their awareness yo make up for having a downside about it, forcing players with dramatically different aspect ratios to behave in different ways.
tl;dr: SOURCE ENGINE ASPECT RATIOS ARE UNBALANCED PLZ NERF; NO 90 IS NOT YOUR FOV YOU PLEBIAN