That is not entirely true, there is some overlap but also enough distinct features on the language level to make a difference. For example object oriented features in OCaml are quite different, OCaml has a more powerful module system (functors), it has open polymorphic variants, extensible syntax. F# has also a number of features that Ocaml hasn't, but it is definitely not true that one is the superset of the other.

The runtime situation is arguably also a matter of preference, Ocaml compiles down to fairly efficient machine code and has a well tuned garbage collector, whereas F# at least initially had to rely on a garbage collector that was tuned for a different use case.