Dimensions change for Laptop tho PC

I am trying to carve parts from both sides. I aligned the blanks (1x4x25" Pine) so I could use the anchor holes that have 75mm spacing. They are round for the most part so the center when I turn the parts over they should be aligned.
Tonight I decided to do all 8 parts at once. On muy laptop, I set up and verified the parts were designed and aligned correctly. The first pass is the center alignment hole 5mm(0.196 inches). Since there are 3 bit to change on the first side I took a while. After I turned the parts over I noticed I didn’t have my bord square. It should have been a problem because of how holes are for the alignment.
Anyway, after I looked closer I saw the center hold was about 1/10 an inch. I just figured I mad a mistake in my design. I was on the PC in the garage that I almost never change the design from. Something was wrong!
The PC in the garage and my Laptop in the house have different Dimensions!
Why is this?