i don't think there is an excuse for them to not be 18"...but the other dimension is dependant on the actual width of the fabric- since to cut a fat quarter you first cut a 1/2 yard...18" then you cut at the fold---so if the fabric is 44" wide you will have 22"...if the fabric is only 36" wide the fq would measure 18"x18"
40" wide = 20" x 18"...
the 18" is (should be the constant)