Can’t figure out backup pattern

I’m trying to have 1 full, 9 differentials, and repeat but keep the last 30 files.

I have the following Custom scheme:

- Differential
- Create full version after 9 differential versions
- Store no more than 3 recent version chains

Here’s what I got so far:

(C)_full_b1_s1_v1.tib
(C)_diff_b1_s2_v1.tib
(C)_diff_b1_s3_v1.tib
(C)_diff_b1_s4_v1.tib
(C)_diff_b1_s5_v1.tib
(C)_full_b2_s1_v1.tib

Why did I get a full_b2?