The reason of F# guys did things different from OCaml is not because system-level programming but because F# is a language designed for the .NET ecosystem which imposes specific type constrains. F# language was not specifically designed for systems-level programming.

Again, the semantics of Int is different but the semantics in OCaml of Int32 and Int64 is the same/standard. So you have 3 types: int, Int32 and Int64 and it is an static typed language.