Weapons have two models - the in-view model and the world model (seen in 3rd person). You can set a separate FOV to render the in-view model if you'd like. Our artists find it easier to get the weapons to look the way they want with this feature.
edit: from the source faq on collective
i never really paid attention to this answer, but from what it says it sounds like you can set a different fov to the first person weapon compared to the fov of the screen, am i reading this right? could you do this in hl1? i've heard complaints about weapons not looking right due to the fov so will this fix the issue?