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.
Extra preface. I’m not sure what method you used for checking the diameter so this might not apply and I’m not calling you out. However, I wanted post this as most will think to use calipers or the like to check a tool’s diameter.
You can only measure a single flute tool with an optical system (swept diameter) or by the cut (sort of). They can’t be measured with calipers. This is because material has to ground away behind the flute or the unground material will rub/grind into the cut. Another way to put this is if a single flute did measure the cutting diameter from the edge of the flute to 180° from the edge it would constantly be slamming smooth carbide into the material before the flute cut again.
In general it’s hard to correctly measure any tool accurately with calipers as the cut diameter is always going to be dictated by the largest flute (from center). That’s on top of possible damage or the change in cutting diameter due to runout. Although, those aren’t the tools fault.
More on this if interested:
Actual size of the #102 endmill
To be clear I’m not saying that an incorrect sized tool isn’t your issue. Just that it’s hard to measure a single flute cutter. Probably the easiest way without special equipment is to cut a slot and measure the width of the cut. That will be added to by runout and substracted by material compression. But it’s a reasonable way to do it.
Hope that’s useful. Let me know if there’s something I can help with.