Hey Mike;
I am not 100% sure. I was working in TV Broadcasting when this standard was presented but My guess would be for reference to the then standard of NTSC (this is pre HD stuff).
In order for the new "Digital Video Tape Recorders" (HD in an SD world) and all the newer Digital Audio stuff that was happening at that time (very exciting I must say), to stay referenced to the analogue Time Code and analogue a "phase shift of 90 degrees was required to keep all the little soldiers marching in time with each other. The "Phase Shift" was to bring the 2 together, allowing for latency.
Just a guess. If you wish to know for sure I can dig a little deeper and see if some of my fellow retired TV Broadcasters remember this stuff?
It may be all a part of history ad may be lost in the cobwebs of their brains.
Edit.
Here is some reading material on the DTRS code.
http://www.silentway.com/definitive-dtr ... de-submenu.
Some lite bedtime reading!
Cheers
Bruce