For cutting external geometry, I would expect lost motion to give a smaller than nominal dimension. It would appear to me that your running larger than nominal but the same in both x and y could be caused by tool deflection. The fact that your dimensions increase as you go down the block would support that.premise. I would try again with a new end mill.
To determine whether this is a cutter problem or a problem with the mill, you could sweep a 1-2-3 block with your Haimer as a test. You will have essentially zero deflection force that way. You could also mill a rectangular pocket and check the dimensions that way. If cutter deflection is the cause, you will see smaller dimensions.
RJ