formulas not giving correct total

formulas not giving correct total

I have a formula field in my mileage expense entry form that enters the product of the miles driven and a constant mileage rate (50.5 cents).

When I use the 'sum' feature in the creator view mode, I get an incorrect total. In the attached pic, you'll see that I have two entries of 21.51. This should give a total of 43.02, but instead shows 42.

Any thoughts?