Hopefully I can help you out on this one.
The top 20 list is not who goes to the most races. Only your 6 best scores count. I got into the top 5 with only 6 scores (3 weekends). I am now in 6th with 8 scores as some people moved up.
Since you only get points for your main it is a lot more economical for me to race ABA. In the NBL you have to get 1st in every single moto and the main to get the best points. One bobble in any of the 3 motos and your score is shot. In the ABA you can take 8th in the first moto (fall, equipment problem etc.), come back in the second moto and transfer, win the main and get the 1st place points.
Someone always gets the 1st place points at an ABA race. Very few do at the NBL nationals, it is hard to get a perfect and it gets them all to come back and try again at the next national.
Since the ABA Grands counts for double national points you can really make up some ground there if you make the main. How things fall out for moto selection has no impact, unlike the NBL Grands where moto selection can make or break you.
The ABA only plates the top 10 per age group (called NAG for short, National Age Group). This makes it much harder to get one of those plates and they are very prestigious. The plate is grey.
They also award what is called national numbers. It is your best 6 scores compared to every other racer in your gender and tire size class. The plate is black.
Next is the ROC (Race of Champions) which is the Friday before the Grands. The winner of that race gets a gold #1 plate that they can run anywhere for the next year.
Then there is the Redline Cup which has a series in each region. At the final, the winner gets a red #1 plate that they can run for the next year.
The district points give you a number in your district that you can run anywhere, including at the nationals. If you are district #1, that is your number and you run it everywhere. This is a white plate.
bitd, we used to be able to run our NBL state plates in any state. This is similar to that but on a total point accumulation. This one is for the people that race a ton of local events. Similar to the state highpoints in the NBL, but the NBL one is more scewed towards national riders. A perfect at an NBL national gives you 800 local points, where in the ABA it is only 300. This gives local only riders a chance to compete for the district plates and not get completely blown out of the water.
Let me know if that helps or if there is anything else I can help with.