You will see this when you have less shift time remaining than drive time.
Your drive time is limited by how many hours you have remaining of your 14-hour shift. If you have spent a lot of time in On Duty Not Driving or Yard Move, then you might not have enough shift left to Drive all 11 hours you are allowed daily.
For example, let's say you spend an hour getting loaded and doing your pre-trip, then you drive 4 hours to get unloaded. You're stuck there waiting for 4 hours before they can get you unloaded. By the time you can take off for your next load, you've used 9 hours of your 14, and only 4 hours of your 11.
Even though you technically have 7 hours of Drive time remaining, you only have 5 hours of Shift time left in which you can legally drive.
To make sure you don't violate the 14-hour rule, the app will only show 5 hours of Drive time remaining. The Drive (Shift Limited) and Shift clocks on the Available Hours screen will always show the same time.
If you find yourself in this situation and you need more time, and your truck has a sleeper, consider using the sleeper berth provision to do a split shift: How does the app calculate split shifts?