That's not actually correct.
The "play" in the end of a tape measure is intentional. If you hook the end on something and pull it will be accurate and when you push it against something it will also be accurate because the end slides to account for it's own thickness.
Now, if you were to drop your tape measure and bend the end it would no longer be accurate. It happens.

Great catch! Welcome to the sport of flyfishing.
Enjoy every fish, as some days they're hard to find.
At the risk of sounding judgy, I encourage you to look into safe handling of fish that you are returning to the water. Trout are pretty fragile creatures and don't hold up to the kind of handling that many other species do.
Keep up the good work and have fun out there.