GFI outlets and breakers are just like anything else, they are made of different qualities. This affects their performace. Another issue is they are rated for different amounts of amps. Some are 20amps like a normal breaker load, while others are only rated for 15amps. I would suspect some of the cheaper ones are even less than that. And like most things, you can have one out of a batch of them that doesn't perform correctly. I have a electrical outlet panel for my 75g with about 8 sets of outlets, all GFI. There is one set that always trips whenever I plug in something. The reason I installed so many GFI outlets instead of just one breaker or one GFI at the beginning of the circuit is that they will trip independently. That way if while I'm out of town or away at work for the day and one piece of equipment trips the outlet, it will shut off while the rest of my equipment will stay on.