That capital D is not the quality control increment (I’m sure someone, like
@TimFL1 ,knows what it’s really called). The capital D is the minor build version number, meaning that this is the 4th minor version of iOS 15 (the earlier ones being 15.0, 15.1 and 15.2 ).
What you are thinking of is the lower case letter that appears at the end of a build number for a beta. Those represent how many times the build had to be compiled before QC allows it to be released. That’s why you theoretically want those to be letters earlier in the alphabet, as it means the build had to be compiled fewer times before meeting quality control standards.
As you will note, the lower case letter is omitted from the release candidate build number.
See this article for more info:
https://tidbits.com/2020/07/08/how-to-decode-apple-version-and-build-numbers/