The built-in FPS counter is far too inaccurate

After noticing that the FPS counter sometimes drops below or even increases beyond the refresh rate of my system (with V-Sync on, by the way) according to SRB2, I decided to check with a more accurate FPS counter, and after checking with MangoHUD, I noticed that the FPS counter that SRB2 uses is way off! See the picture below, where SRB2 reports an FPS at 128 while MangoHUD reports 144 FPS:

The bottom-right is, as you all know, the built-in FPS counter, while the top-left is MangoHUD. This drastic difference renders the built-in FPS counter useless, which is unfortunate since it makes you think that the game runs better or worse that it really does. I haven't looked into why it's so inaccurate, but I think this can simply improve by just doing what an fps counter should do: count the frames in a second. Sure, it's not as responsive, since you only get to see the framerate once every second, but it's the most accurate way of measuring the framerate. Alternatively, a good compromise would be to just count 1/10 of a second and multiplying with 10, which makes it more responsive while still remaining pretty accurate.