There are two possibilities here
- The ride data is out of sync with the video
- Your trainer needs a spindown
If you experience that "out of sync" feeling, try a different ride. If that ride seems fine, you've stumbled across some bad data in the ride. Please go to the community section of these support pages and let us know about the problem.
If other rides are "out" too, you definitely need to calibrate your trainer by doing a spindown in it's utility app.
Videos can be deceptive, they often look flatter than they are in real life and it's also hard to tell the difference between "steep slope then steeper" and "downhill then uphill"
IMPORTANT NOTE ON SPINDOWNS AND CALIBRATION
Once you have done a spindown, it's really important to then quit that app before you use FulGaz. If you don't do this, your trainer will still be paired to that app and FulGaz will not work. In some cases you will also need to "forget" the trainer in the utility app.