Did you measure the current periodically during this test?
As the voltage decreases the current will decrease. As the bulb gets hot the resistance will increase, causing the current to decrease as well. I do not believe you were drawing 6.1A throughout the whole 10 hour test. It would have been slightly decreasing with the voltage.
If you have the time, repeat this test taking simultaneous voltage and current readings at a periodic interval noting the time of each data point. Maybe every 1/2 hour or once and hour for the data record rate. From this data you can use Excel to calculate and graph actual depletion vs time.
