What is the value of Availability at the point (5193.6.....) found by NMaximize? Is it the value shown by NMaximize? I'd would bet the point returned by NMaximize actually exists.
The plot might not be sampled at enough intervals to show the true maximum if the function has some erratic behavior. I'd bet your problem is with Plot and not NMaximize.
Plots are good tools for intuition, but they often are really deceiving.