The rule for numbering software release versions is: there is no rule.

In FFXIV version 1, they used: 1.1, 1.2, ... 1.9, 1.10, 1.11, etc. Minor "fix-it" releases were marked with letters. So patch 1.19b meant fix #2 of update 19 of version 1. The order was 1.19, 1.19a, 1.19b. ARR may or may not continue with this.