as I was getting ready to do a stiffening upgrade of the x axis i noticed the same thing and when I moved the carriage across the x, i could see the v wheels riding up slightly on the maker slide as the two of them became spaced further apart. I have a set of machined brass set up bars that I use in my woodworking and with the 3/16" bar, I set each end at that spacing and discovered the center to be narrower by as much as 1/32".
I acquired a 3/16" piece of aluminum flat stock 1 3/4" x 39" and did the stiffening upgrade.
i also added angle brackets to the y rails in the center and anchored them to the wasteboard.
After finally gettign my new 48v 300w spindle installed and wired up, I lettered a sign in white oak using a 1/4" core box bit to a depth of 1/8" in 3 passes with virtually no chatter or tearout.
I am stoked that this mod paid off!
Here is the thread that gives real good info on this mod.