HP-35s Order of arguments with RPN

[erazor]

Hello,
I'm reading the manual for the HP-35s and I'm confused with RPN input mode on page 1-20.

The top display line of the calculator shows the y register and the lower line shows the x register (Page 2-2).

Now page 1-20 shows how to enter 6C4 in both modes.
In RPN the 6 stands in Y register and 4 in X when nCr is pressed which would translate to nCr(4,6) if the function notation is f(x,y). Also if this is right the xth root of y exception mentioned on the page would be none.
But in ALG mode nCr(6,4) is entered, which would translate to f(y,x).

What am I missing in my understanding?