Good point @Mike, I didn’t even think about out-of-square. Here’s a link to a forum where we were helping to troubleshoot a spindle that wasn’t square to the board: Having an issue with Y axis not cutting square and Z axis Depth - #7 by RobertA_Rieke
It basically comes down to using the widest bit possible to carve a shallow pocket, and then shimming the Z axis (and squaring the X axis) based on the resulting sawtooth wave in the board. The large bit enhances out-of-square, so if you can get that to cut flat then you’ll never see the results with a small bit.