Check your rulers against each other - the one you've got in the pic and the one you used to cut - they may not be the same.
I learned a valuable lesson about that some 20 years ago. I used the measurements on my mat to cut strips and the measurements on the ruler to cut those strips into squares. Needless to say, they didn't turn out square!
If I use the mat to measure when I start cutting, that's what I use all the way through. If I start with the ruler, I keep with the ruler.
It's amazing how much a few threads lost can turn into inches lost by the end of the project.
Probably not what the problem is, so just ignore me after laughing.
:)