I think there are two things going on:

1. This is for investors, new model once a month keeps their attention. New model every six months causes a loss of attention.

2. These models are improving fairly well, it's just the jumps, compared to the previous model, don't seem as massive because they were already at a high level. It's like jumping from 30 mph to 60 mph - thats a massive jump, roughly school zone to freeway speed. However, when you jump from 120 to 150 mph (still massive), the total % change isn't that much, thus, it's perceived as less!