Can someone explain edible dosage?

While not new to smoking, I am a newly registered medical patient and i am just learning about edibles. I have been experimenting with different types and strengths and I have one major question...
Why is it that I can eat 1/2 of a 60mg brownie and feel practically nothing while a 20 mg hard candy has me feeling great and still yet a 10 mg cookie has me staring at the walls? This seems backward!
All were the same brand, purchased on the same date, at the same pharmacy, consumed on an empty stomach near the same time of day, all with dramatically different effects! I can understand how the hard candy might be absorbed quicker, but not why that would make a difference in the overall potency. Is it common for an individual to have different tolerances to different products or even to different brands?