There are basically two systems for numbering software versions, one where the numbers are a single number with a decimal (what you're thinking) and one where the dots just separate different numbers (what Raikki is talking about). In the former you might go from 1.9 to 1.91, but in the latter the revision following 1.9 might be 1.10 or even 1.9.1.
Technically we don't know how FFXIV versions are numbered because the numbering didn't start until 1.15a after the change in development team members. The "0" they use in 1.20 would suggest we're using the system Raikki is talking about. It will be quite a while before we get 2.x revisions that show for sure how they're numbering them.
tl;dr: Take a software engineering class.