Sloppy hole after drilling on lathe

If the tip-corner of the drill bit is heavily loaded, the flutes will bend backwards under the force and drill bigger than the measure diameter of the drill bit.

But::
a) drill bits do not make round holes
b) drill bits to not make accurately sized holes.
 
As other have noted, use the next smallest to open the hole and finish with the desired size. One tip is to dull the drill tips just a little bit. Can help reduce the likelihood of drilling oversize.
 
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