My aircraft hud and the nature of angles

I’m making a fighter-style hud add-on right now, and the visual part of it seems to be coming along nicely. However, I’ve noticed that the angles of Garry’s Mod entities is not quite what I need. It seems that as the pitch of an entity increases, the upper and lower limits of the roll decreases, and it looks like it works the same visa versa. If I add roll to an entity while it’s pitched up, the resulting angle will be a portion of the roll I added, and the pitch will be less. Of course, this isn’t what I need for a fighter hud, as the pitch and roll could be both anywhere from -90 to 90 at any time. Is this the nature of how source engine works, and does anyone have any ideas of how to counteract this, or am I just being derpy and using the wrong function (getAngles())?