If you've been using your ruler for a long time, check to make sure that the edge you're cutting against is still straight.
I've used mine so much that it has a curve in the middle of it - which may help explain why my strips were uneven!
I'd move my ruler down - I knew I had cut "straight" - and I had fabric sticking out in the middle of my ruler - FINALLY realized that the ruler is worn down in the center.
This may or not bother you-all. It bothered me. Now - what do I do with that ruler? The other side is worn, too, but not as much.