Usual preface, I’m with PreciseBits. So while I try to only post general information take everything I say with the understanding that I have a bias.
Without knowing more I’d guess that you are dealing with deflection (bending). It would be hard to say without at least more info on the chipload and tool (feed and speed plus flute count and general tool geometry). However, generally speaking the cutting forces cause the tool and machine to deflect as you are cutting creating dimensional inaccuracies (everything is a spring). You could try doing a cleanup/spring pass to try and get it back in spec with the assumption that the code and tooling is correct. It’s going to be harder with easel as I don’t believe they have cut direction control (been a long time since I checked though).
Some other options would be to reduce the cutting forces. Cutting forces are more or less proportional from chipload (feed), stepover, and pass depth (though in different directions). You can actually get in a worse situation with reducing chipload if you are rubbing where more force and heat get generated. So the easiest thing to do is to cut the pass depth. For stepover if you are slotting it’s your tool diameter. So you end up in situation where you have to decide between a stronger tool or a lower cutting force. Until you reach the limits of the machine if you can use a bigger tool that will typically win on the rigidity side.
One last note. I don’t recommend ever physically measuring a tool. See here:
Your best bet is to cut a slot and get your actual cutting diameter that will include things like your runout and material compression to know what you are actually cutting.
Hope that’s useful. Let me know if there’s something I can help with.