I have a table like this

a b c
6.75 -.75
6.875 -1.25
7.0 -2.0
7.125 -2.125
7.375 -2.75
7.5 -3.0

Everything works fine on my spreadsheet until I change from 7 to 7.125. It
returns the same value(-2.0). I don't know what I am doing wrong. Please help.
Landon