Did the pattern say to use fat quarters? If so, the 21 inch dimension may have been to make sure you cut the fabric width of fabric.
If you are cutting 2.5 inch slices from your pieced strips - you should be fine - You should get 8 "slices" with a teeny bit left over.
(I know that when I cut a 2-1/2 inch strip - it is actually about 1/64 of an inch larger than that due to the space between my ruler edge and where the rotary cutter makes its cut and how I place my ruler.)