Tool holder mounting holes diameter question

just stumbled over a small detail:

Here, the diameter of the holes to be drilled in the Z-rail tubes is given as 8mm diameter. All adapters, tool holders, drilling templates that I saw - and used - say 4mm diameter. I made 4mm holes, and they fit perfectly. Where is my mental gap?

You don’t want the tubes actually touching the screws so you over size the tube holes as pass though holes and clamp the holders on with the nut traps. Make sense? This helps give a little wiggle room for tool alignment as well.