Tiger Still on 8A series

I've expressed before an interest in Apple build number conventions.

The latest Mac OS X 1.4 "Tiger" build number is apparently 8A425.

What is unusual is that they are still on the "A" series unlike previous releases:

• 10.0.0 was 4K78
• 10.1.0 was 5G64
• 10.2.0 was 6C115
• 10.3.0 was 7B85

Interestingly, the middle letter has consistently been lower each major release.

I'd still love to know what triggers a change in the middle letter (and why that trigger never happened in Tiger).