C-language stored function and float4 type

Hello! I have C-language stored function like this:

#include <postgres.h>
#include <fmgr.h>

#ifdef PG_MODULE_MAGIC
     PG_MODULE_MAGIC;
#endif

PG_FUNCTION_INFO_V1(testfunc);

Datum testfunc(PG_FUNCTION_ARGS)
{
     float4 x = PG_GETARG_FLOAT4(0);
     float4 y = PG_GETARG_FLOAT4(1);

     PG_RETURN_FLOAT4(x + y);
}

CREATE FUNCTION testfunc ( x real, y real )
RETURNS real AS
'testfunc'
LANGUAGE C STRICT;

If i use this function (SELECT testfunc ( 0.1, 0.2 )) I get result with
type "double precision". I tried used float4 but the result was the
same. What is wrong?