Why is there a difference between the procedural track heading and final approach course at Fort Hood (KGRK)?
It's really only confusing because they are within one degree of each other, but this actually happens quite a bit with feeder routes.
The 151 Degree Course NW of STARN
The 151 degree course (and the OKEST intersection) is only used if you are using AGJ as the Initial Approach Fix (IAF). This is the course that you would fly from OKEST towards the GRK VOR (on the GRK 331 degree radial) in order to get to STARN.  After reaching STARN, you would fly the 150 degree bearing outbound from STARN and the rest of the approach using STARN to navigate.
The 150 Degree Bearing NW of STARN
If you were to use STARN as the IAF instead of AGJ, you would fly the procedure turn on the 150 degree bearing from it in order to get yourself established on the correct inbound course to fly the rest of the approach.  In this case, you would never use GRK at all.