I haven't read all through this thread but I would guess that you have a high-paying niche like web hosting or finance.
High paying niches will pay out well. Once you find these niches, your adsense income will increase rapidly. The best way to find the best niches is to diversify yourself as quickly as you can. I find that whenever I get into a new niche, my adsense income increases quickly.
Congragulations! That is great! I am sure you are floating in the air right now! Keep up the great work