I'm not entirely sure, but zooming closer to gear teeth interaction, it looks like cogs aren't aligning perfectly like they do in real life? I'm viewing this in firefox, so that might be a reason as well.
Here there are off a bit because 1. I didn't get the sizes absolutely right as I modeled it in Blender and not in a real CAD (like Gearotic) and 2. The gear simulation is iterative so some floating point error accumulates over time.