Phyzix Object - Pendulum Example / Problem

I have created a swinging pendulum by modifying one of the examples that came with the Phyzix object. All of the fricion settings are set to zero (as far as I can tell). The simulation does a nice job of showing a pendulum swinging.

Now the problem. Theoretically, a pendulum should keep the same time to complete one complete periodic motion (the period), no matter what the amplitude or distance from the center point of the swing it is started.

I carefully measured this time as follows using the pendulum simulation:
length = 300 amplitude = 50 avg period for 10 trials = 10.88 sec
length = 300 amplitude = 200 ave period for 10 trials = 11.23
This is a 3.11 percent error. While it is great to look at, it is not good enough as a demonstration of the physics principle mentioned above.

Is there anything wrong with the way I set up the Phyzix object that would account for this error or is this just within the limits of the objects real world accuracy?

I will upload a copy of the file.

Any help appreciated.

Steve