Som apps don't obey Quicktime rules, some default straight to Rec709, others you can set up for whatever colorspace you want to monitor in.
This really started being a bigtime issue when Apple COLOR was introduced, because it automatically switched into Rec709 (2.2) while all of FCP was optimized for graphics monitors -- and relied on external Video I/O cards to fix the scaling issues for broadcast display. So what you played on the desktop (under Quicktime, roughly in sRGB 1.8) would not match what you were seeing displayed under broadcast rules. Close-ish... but either whoa whoa whoa way too dark, or the other way around... and then there is the issue of full scale 0-1024 10-bit vs linear video 64-940, which, when you mix those up are very hard to fix... because its not just of matter of fixing the black/gain, the gamma has been incorrectly mapped, as well, so there is no linear correspondence along the grey scale.
"I always pass on free advice -- its never of any use to me" Oscar Wilde.