Gross price changes in ML81N

Hi Guys,

   While doing SES , gross price changes for some amounts.   PO is in USD for quantity 1 . While doing SES for 0.222 / 0.224 / 0.226 , no errors crops ,, but when I  enter quantity of 0.223 my gross price changes and I get error " Net price cannot be greater than po net price " . My gross price increased by 0.02. Why does gross price increase when I enter quantity of 0.223 and does not change when I enter 0.222...?

Regards,

SK