When overloading operators such as (+) as global functions, there need to be two separate arguments. Overloading (+) as a static member method only works with a tuple argument. Is it as arbitrary as it seems to me, or I am missing the reason?

type VecRec = {dx: float; dy: float}
let (<+>) (v1: VecRec, v2: VecRec) = {dx=v1.dx + v2.dx; dy=v1.dy + v2.dy}
let (>+<) (v1:VecRec) (v2:VecRec) = {dx=v1.dx + v2.dx; dy=v1.dy + v2.dy}

type VecClass(x: float, y: float) =
member v.X = x
member v.Y = y

static member (+) (v1: VecClass, v2:VecClass) = VecClass(v1.X + v2.X, v1.Y + v2.Y)

> {dx=1.;dy=2.} <+> {dx=-1.;dy=-2.};;

{dx=1.;dy=2.} <+> {dx=-1.;dy=-2.};;
^^^^^^^^^^^^^

error FS0001: This expression was expected to have type
VecRec * VecRec
but here has type
VecRec
> {dx=1.;dy=2.} >+< {dx = -1.0 ; dy = -2.0};;
val it : VecRec = {dx = 0.0;
dy = 0.0;}
> VecClass(1.0,2.0) + VecClass(-1.0,-2.0);;
val it : VecClass = FSI_0011+VecClass {X = 0.0;
Y = 0.0;}