Running test board carve

While carving my test carve , I was playing with different bits and carve’s .
Trying to made 3/8 deep hole 1/4" DIA to use for 1/4" shank bits using a 1/16" bit.
setting at 0.250 x0.250 and using the fill method. I makes the hole about 0.280 .
Any reason why it would not make it the 0.250 size ?

Thank you

What calibration should be checked ?