Welcome to Quicktime gamma hell. Macs have a native gamma of 1.8, PCs have a native gamma of 2.2. Quicktime automatically assumes a screen gamma of 1.8 on Macs, and adjusts playback levels accordingly. It’s an infuriating “feature” that can’t currently be turned off/defeated. Search the forums for “Quicktime gamma” and enjoy the fun. Let’s hope that Apple finally addresses this issue in the next release of Quicktime. 1.8 gamma is SO last century, and SO print-centric. Come on Apple, get with the program already.
There’s a program called Quicktime Gamma Stripper that supposedly solves the problem, but it only runs on Windows – go figure. Here’s the link:
https://support.franticfilms.com/wb/default.asp?action=9&fid=14&read=74
Good luck!