Parsing method which the generated parser employs is Recursive Descent Parsing.
And the behavior of the parsers recursive and A is a little bit complicated.
The parser recursive matches the sequence of 'a' whose length is 2^n-1 such as 1, 3, 7 and 15.
Anyway, I'm going to write more documentation of ctpg within a few days.
I hope it'll help you.