Did you place them on a 1X1 hole? It is required for the issue to occurs.Else they do burn in the same amount of time.
Edit: Wait, I tried burning several of them as you did, and it did burn correctly. It seems that it never takes the same amounts of time to burn.
Strange, it works 100% of the times for me, I added screenshots maybe it will help.
Did you place them on a 1X1 hole? It is required for the issue to occurs.
Else they do burn in the same amount of time.
Edit: Wait, I tried burning several of them as you did, and it did burn correctly. It seems that it never takes the same amounts of time to burn.