Most likely this is variability in the material.
The unfortunate truth is that when you buy stuff that isn’t designed for lasering or to have uniform thickness/composition you’re always going to have to test.
I tend to run a material test on every board and write the successful settings directly on the back of the material or color code the edges if I have a lot (like a stack of Baltic birch ply)
My testing method is pretty well known but if you haven’t seen it here you go, check out #6:
Once you have the job set up testing a new material is really fast and easy and consumes only a very small amount. It’s the best way to be sure, the time effort and materials used pay for itself by avoiding even one botched job.