R. I am not an expert on this but mine seems to fit ok. The poles are 12 ft so that the actual useable space is ten. I've never measured the width or length of the sheets. I just seem to have more than I need.
I have only used the white muslin from my set, and I only used 3 of the four top bars. The muslin was a little longer than the 9' but probably not 10'. I recall reading some reviews before I bought that indicated the 10' advertised was considered a "nominal" measurement, or in other words, an estimate. I believe the muslins start out 10' and then shrink when washed perhaps, or when end seams are sewn. I wasn't worried about it. I can see where you might find it disturbing if you didn't read about it before purchase. I am happy with my 10' nominal. Greg M.