Why would an experimental shallow wave be faster than a theoretical shallow wave?

I did an experiment where I looked at the relationship between water depth and wave speed for shallow water waves. My data shows that the waves I created were faster than the theoretical values. Is there a theory/concept that could explain why this happened?

See video of trial here:
https://drive.google.com/file/d/1PBXgW9d8J4NrCa86lxeekI1xPIe5MWsx/view?usp=sharing

In the experiment I lifted the side of a fish tank filled with water and placed it back. I measured the time it would take for the wave to move from end of the tank to the other. I only took data for first few wavelengths, where there was little wave interference.