...okay, well, first of all, it's probably just a driver error or something, or else minecraft detected the better GPU and increased its settings.
Second, running at 3-400 fps is absolutely horrible, because it's taxing the GPU and producing a lot of heat for absolutely no reason.
Your monitor can (most likely) not display anything more than 60fps... which means running a little above that so you don't dip below it is fine, but running at 400 fps is wasteful and possibly harmful to the computer.