Sorry, I was unclear. Run Availability on the actual value returned by NMaximize.
Mathematica clips numbers when displaying them. This is so that the output stays readable. For example:
N[Pi]
3.14159
InputForm@%
3.141592653589793
InputForm shows you the actual output of a command. Try using it to see what the rest of the decimal values are in (5193.6.....)
InputForm[NMaximize[Availability[...],....]]
Is the value for Availability different when these digits are used?
I'm guessing that this difference in value is a numerical instability in the definition of Availability. Are you expecting availability to be smooth or jagged?