Yea that causes it's own problems. Cause the fence texture is probably used in other parts of the map but you don't want you have the same graffiti everywhere.
And if you create a specific texture for that one piece of fence it adds to unnecessary memory. So it's better to just have another 'static' (so to speak) that is the graffiti placed next to the fence.
Tbh I only think this is how it works, I don't know for sure.