Mary Ellen Hopkins - and several other authors - suggest doing a test with several strips of fabric to see how your cutting and sewing "measures up"
I cut strips with a rotary ruler - they end up being a smidge wider than if I cut strips with the June Tailor shape cutter.
I use "my" 1/4 inch way of sewing.
Anyway - the "test/sample" goes like this:
For example:
Cut three strips of fabric 2.5 inches wide by 6.5 inches long.
Sew them together with "your" 1/4 inch seam.
Press
Measure - the "expected" size of the unit is 6.5 x 6.5 inches
It is another of those "prep" steps that saves one HOURS of time and aggravation later.
On your "off" block - if all your other blocks are the correct size, you might have cut something wrong on the off/odd one -