Yes, I know the same feeling (and have remarked on it once or twice on the DNB ML) - it's very strange how over a day or two one can suddenly jump 10 or 20% on a level and have a feeling that eg. suddenly D4B is clear and comprehensible, while before only D3B was and D4B was a murky mystery one had difficulty keeping in one's head.

On the other hand - D8B? Dammit! I've been at n-backing for something like 2 years now, and have been stuck on D4B for months. You, Jonathan, and Will just go straight to D4B or D8B within a few months with ease. I must be doing something wrong.

(On a sidenote, as in the FAQ, I ask people for their negative or null reports as well as their positive ones. This thread is unusual in 2 null reports to 1 positive, but I'm sure there are more LWers who've tried!)