You've not allowed for the unequal force when cutting of centre.

Ideally, you should calculate the static load on each bearing.
Then calculate the force applied in the worst case scenario of the maximum cutting force being applied at one side of the gantry i.e. all the cutting force being applied through just 2 bearings.

In an ideal world, where the gantry is completely flex free, the cutting force would be transmitted through all 4 bearings, but in the real world it's not going to happen. Of course, some of the force will still be transmitted through the opposite side, but without lots of calculations or FEA modeling, it's simpler to use the worst case scenario.