When using the OG slider, the program adjusts the hops to keep the IBU level the same. If you are using the Tinseth IBU model, then the expected IBU will change with the gravity of the wort, so the program tries to keep the IBU on target, thus changing the hop amounts. In actual practice, I think the programmer envisioned the sliders to be used for minor adjustments of target values, so wanted to keep the targets independent and allow the program to adjust the ingredients to suit.
If it were to keep the hop amounts constant, then the IBU would drop as you increase the gravity. So it is a matter of what the programmer deemed most important to him and what he envisioned most people would want.