I was trying to cut something the other night and the hole is set to 21mm however when I preform the cut, it’s only 21mm in diameter. Anyone have thoughts on where I should start?
Start with a calibration. There are a few threads here about it.
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I’ll start with a sarcastic, tounge in cheek reply to your question “Anyone have thoughts on where I should start?”
Answer: Lower your expectations 
The better and proper reply is however:
There are only two reasons why a shape of given size come out with different size.
1 - Machine not fully calibrated (for its intended task)
2 - Mechanical attributes (bit actual size and runout, tool deflection)
Ideally you would first address #2 and have that under control. (Impossible to eliminate all of it, you just need to get it as good as you can/need). If a machined part have a 1mm discrepancy this means you have a 0.5mm deviation per side. If the discrepancy = 1mm for both the 22mm part and say a 220mm part (scaled by a factor of 10) then your cause is runout/tool deflection. If the deviation is closer to 10mm for the 220m part then its a calibration issue.
No machine is immune to (100% with 0.0000000000" precision) backlash / tool deflection / runout so you need to first establish the given deflection for the typical load, once known you can adress #1 to tweak calibration values based on actual #2 effects 