They are made bigger in order to get enough room either side of the slide for the roller ends and furling mechanism. Three inches each side is fairly typical.
Our rig came with the factory slide awnings, but the living room one had perished due to extended exposure to solar radiation. The rig had been parked for about 2 years with the slides deployed and the living room faced south. I paid to have both LR and BR slides replaced, as I didn't feel confident about spending a lot of time on top of a 12' ladder doing it myself.
Frank Damp -Anacortes, WA,(DW- Eileen)
ex-pat Brits (1968) and ex-RVers.