You're confusing the number of frames generated on your computer with the number of frames displayed on your monitor during any given time interval.
With a refresh rate of 75Hz, your monitor will display a new image 75 times per second. If you have a game rendering 150 frames per second, that means each time the monitor changes, you'll be seeing half of two different frames (more or less, depending on the exact timing). Raise the frame rate, and you'll be seeing a smaller fraction of more frames on the same single screen update.
As for the 150 limit, that's imposed by the game engine. There's a way to set the limit higher, but you gain nothing by doing so.