[Apologies for Cross posting]

AACR2 has following Outline:

Part 1: Description
Chapter 1: General rules
Chapters 2 -12: Resource type - specific rules
Chapter 13: Analytic entries

Part 2: Headings, Uniform Titles, References
Chapter 21: Choice of access points
Chapters 22 – 26: Construction of access points

Appendices
A: Capitalisation, B: Abbreviations, C: Numerals, D: Glossary, E: Initial articles

Question:
After Chapter 13 there is Chapter 21. Can some expert explain the reasoning behind this numbering of chapters? Why are 14-20 not used for numbering the chapters?

Thanks & regards
Salman Haider