If you set it to 75 Hz and it works, then that's it. It works. If it isn't letting you, then, yes, HDMI is limiting you. I know that most HDMI revisions are limited to 1080p @ 60 Hz.
Also, I'm not sure what jitpublisher is saying. Refresh rate definitely does something. It's basically how many FPS your monitor can display. If you have 120 FPS in a game, but your refresh rate is 60 Hz, you can only see 60 FPS. If it's 75 Hz, you can only see 75 FPS, and so on.