gcc output on cygwin using stack space outside stack frame

I was looking at the assembly output of 'objdump -S' and noticed something strange. This was on cygwin/x86_64 v. 3.1.5 with gcc 9.3.0 on Windows 10.

Here is the assembly output of a particular function (the function is not useful and is merely illustrative of the problem):

u_int64_t returnit(u_int64_t x) {   1004010b9:   55                      push   rbp   1004010ba:   48 89 e5                mov    rbp,rsp   1004010bd:   48 83 ec 10             sub    rsp,0x10   1004010c1:   48 89 4d 10             mov    QWORD PTR [rbp+0x10],rcx        u_int64_t a = 1;   1004010c5:   48 c7 45 f8 01 00 00    mov    QWORD PTR [rbp-0x8],0x1   1004010cc:   00        return a + x;   1004010cd:   48 8b 55 f8             mov    rdx,QWORD PTR [rbp-0x8]   1004010d1:   48 8b 45 10             mov    rax,QWORD PTR [rbp+0x10]   1004010d5:   48 01 d0                add    rax,rdx}   1004010d8:   48 83 c4 10             add    rsp,0x10   1004010dc:   5d                      pop    rbp   1004010dd:   c3                      ret

Almost everything looks normal: set up the stack frame, with extra space for the local variable, and copy the passed argument ("x", in register rcx) to a position on the stack.

Here's the part that seems odd:

mov    QWORD PTR [rbp+0x10],rcx

It's copying the contents of rcx OUTSIDE the current stack frame. Local variable(s) are stored in the current stack frame, as they should be.

I tried this on an older installation of cygwin (32-bit, v. 2.9.0 with gcc 6.4.0) and it behaved the same way.

I also tried this on other platforms - an older ubuntu linux liveboot with kernel 4.4.0 and gcc 5.3.1, and a FreeBSD 12.1 box with clang 8.0.1, both 64-bit - and they do what one would expect, copying the value of the argument passed in a register inside the local stack frame. For example, here's the relevant line on FreeBSD (it uses rdi instead of rcx):

2012e8:       89 7d fc                mov    DWORD PTR [rbp-0x4],edi

Is there some particular reason it's done this way on cygwin?