I tried to extend the standard Set module with new operations. I got 
error messages about type incompatibilities (the Set.S.t as exposed by 
my implementation and Set.S.t used by functions from the original Set). 
I have reduced my code to the following small example:

module Set = struct
   module Make (Ord : Set.OrderedType) = struct
     module Set = Set.Make(Ord)
     include Set
   end
end

module OrdChar = struct type t = char let compare = compare end
module Raw1 = Set.Make (OrdChar)
module Raw2 = Set.Make (struct type t = char let compare = compare end)

let aaa (aa : Raw1.t) (bb : Raw1.Set.t) = (aa = bb)
let aaa (aa : Raw2.t) (bb : Raw2.Set.t) = (aa = bb)

Only the last line results in an error:
Error: This expression has type Raw2.Set.t but is here used with type Raw2.t

All the rest of the code compiles correctly. It means that types Raw1.t 
and Raw1.Set.t can be unified.

My question is: why these nearly identical statements results in 
different behavior of the type t?

I'd really prefer Raw1 and Raw2 to be identical.
I use ocaml 3.11.0.

Dawid