Question

I did a science experiment on batteries. I ran five brands of AA Alkaline batteries until they would no longer run a small light bulb. I measured the voltage of each battery every hour.

I did this experiment four times using the same type of light bulb and the same brand of batteries from the same four pack of batteries.

The same brand of batteries acted differently during each run.

Sometimes the would spike when they should have dropped. Sometimes they ran significantly longer than there usual pattern.

Why do AA batteries run so differently from each other? Most cost a similar amount of money. All had the same use by date. Why does one battery last six hours and another last 10. Aren't they made of the same chemicals?