Compare your rulers - if they're different, I would not use them both for the same project. I've talked to lots of quilters who will not use the gridded cutting mats for measuring because the grids are not accurate - I've measured mine and I'm happy with it.
I did discover that 2 of my rulers are NOT accurate on 2 sides. Thankfully, I was drawing lines on paper, not cutting fabric, when I discovered this. They're about 1/16th of an inch too large, which WILL add up! Now that I know this, I make sure I don't measure on those sides.