I think the plastic is softer and it slowly scrapes off when the rotary cutter passes by on it.
For all your QS rulers to be bowed is odd.
I checked my QS rulers with a carpenter's square and no bow in my QS rulers not even the older ones. Maybe the way you are holding the cutter to rub the side of the ruler? i don't know your cutting style but my rotary cutter or blade never touches the ruler at all when I cut. When I started quilting I took a ruler and cutting class at a fabric shop taught by Omnigrid. I discovered I was cutting with the rotary cutter wrong. That class has paid for itself many times over the years with less wear on the blade and mats or damage to my rulers.