Did you ate both with empty stomach? Eating weed with full stomach simply doesnt work.
Also, the conditions you used (time of heating, groundness of weed, etc) were the same? It can have a lot of influence in the final results...
And there is the tolerance... if you eat firecrackers during two days in a row, the ones you eat in the second day will seem far weaker than the ones you ate in the first day.