Determining if the saddle fits?

I always hear people talking about how their saddle doesn't fit their horse. I honestly can't say I understand what they're talking about when they say this. Most cowboys and ranch owners that I talked to laugh at this, and I can see why for they just put a thick fleece pad on and the saddle pads it up where it doesn't fit or whatever. But to me it just seems like saddles no matter what fit on a horses whithers and curve around them perfectly, so my question is, how can you tell a saddle DOESN'T fit, for maybe i'm missing something. Like is this obvious to the eye? does anyone have example pictures?