Keyword, slow you can do a lot of iterations by prototyping. When you offer a product for sale you assume you got it right. Any rush to iterate usually comes from 2 things. 1. It's a disappointment and doesn't perform like you expect. 2. The product has been out long enough that sales have leveled or dropped off and you think a new iteration will boost sales again.

Think Blessing vs Blessing 2 vs Blessing 3. The B2 came out within a year of the B1 because it was a disappointment. Whereas the B3 is coming out almost 3 years after the B2.

The Zero hasn't been out for very long. It's hardware has already been tuned by Crin. It's sold reasonably well. While it is entirely possible that it is either a new release or SE with a different tune based on the zero hardware. The typical reasons for something like this don't really apply.