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.
It’s pretty much impossible to grind a cutting diameter to the shank size barring tools with “land” (where part of raw ground shank is left in the flute). It’s why in high accuracy tooling you will see a +/- on tools with a diameter less than the shank size and a +0 -X on tools with diameters listed as shank size. I went more into it here if you are interested:
That said, how are you measuring the tool? It’s almost impossible to get a true measurement with something like calipers and you can pretty easily damage the tool (also went into that on the previous link). Best thing to do is cut a slot and measure the result. That will also include your runout and the like which give you the cutting diameter that more important.
That is pretty far away though (assuming a good measurement). Is it a new tool? you might also be seeing a reduction from use. Which is another issue that you have to deal with if trying to hit tight specs.
Hope that’s useful. Let me know if there’s something I can help with.