A new NET version comes out yearly and the C# and F# versions align with the yearly version but C# is ahead by 4. Semantic versioning is overrated anyway.
It is fun to point out that the VB.NET compiler is still at least 3 versions ahead. As far as I am aware VB.NET hasn't had a major version number bump since 16.0 (around .NET Core 3.x or .NET 5, as I recall), but the VB.NET compiler still also claims the 6 versions before .NET.
C#'s (lucky) 13.0 in .NET 9 is also just partly related to the non-linear version numbering of .NET itself with the long shadow if .NET 4's many minor releases that were also "major", the Core divide, and the eventual "unfork" at 5.0. F#'s 9.0 matching .NET 9 is a fun coincidence, and also just because F# is younger and missed some of the .NET fun like 4.x "minor number is as important as major number" years.
