I don't believe it's supposed to be allowed through, I think you're just lucky
I just finished off my DMAA caps the other day. Can't say I really noticed much of a benefit I can specifically attribute to the DMAA, so yeah who knows. It's just supposed to help you focus (hence why Uni/college students hammer it for their studies), so I suppose if you aren't struggling mentally with something you won't really notice much of a difference. Towards the end of the bottle, I was taking 75mg (3 caps) - supps that used to contain it like 1MR etc only had 25mg/serving, so if there was a specific effect from the stuff I'd like to think I'd be noticing it at 3 times that dosage.
I can absolutely 100% say for certain though that I got significantly more benefit from eating myself stupid the night before and up until 3+ hours prior to training (since I do poorly when I have anything in my stomach). As Birds etc always insist (and nobody disagrees) food and sleep are a much better solution than stims. Unfortunately gorging yourself on tasty shite isn't really much of an option during calorie deficit periods (as I'm on now for the next 4 weeks), so in these situations I definitely understand repeated use - even if only for appetite suppression haha