The other thing that might be happening .... are your seams 1/4"??
If one row has many seams and you happen to be a thread or two wider/narrower than the 1/4", they could add up to quite a difference on the width of a quilt. Likewise, a row with fewer seams would not have as much variance.
Wondering too?? ... are these the patterns from the Row By Row Experience Shop Hop?