Concerning the Attribute Flat

Question

Suppose we have a function f with the Attribute Flat. From my understanding, the following command should return all possible ways of rewriting f[a,b,c] as a function of two arguments at all levels:

Attributes[f] = {Flat};
f[a,b,c] /. f[x_,y_] /; Print[{x,y}] :> Null;

This command evaluates to

{f[a],f[b,c]}
{f[a,b],f[c]}    
{a,f[b,c]}    
{f[a,b],c}    
{f[a],f[b]}    
{a,b}    
{f[b],f[c]}    
{b,c}

My question: why are the level 2 cases {a,f[b]}, {f[a],b}, {b,f[c]}, and {f[c],b} not included? Similarly, when the following command is executed,

Attributes[f] = {Flat};
f[a,b,c,d] /. f[x_,y_] /; Print[{x,y}] :> Null;

why does it return two instances of {f[a,b],f[c,d]}?

I am running Mathematica 11.2 on Ubuntu 18.04.

Answer 1

Please pardon this non-answer but it doesn't fit nicely in a comment.

In version 10.1 under Windows I get different results:

Attributes[f] = {Flat};

f[a, b, c] /. f[x_, y_] /; Print[{x, y}] :> Null;

{f[a],f[b,c]}

{f[a,b],f[c]}

{f[a],f[b]}

{f[b],f[c]}

f[a, b, c, d] /. f[x_, y_] /; Print[{x, y}] :> Null;

{f[a],f[b,c,d]}

{f[a,b],f[c,d]}

{f[a,b,c],f[d]}

{f[a],f[b,c]}

{f[a,b],f[c]}

{f[b],f[c,d]}

{f[b,c],f[d]}

{f[a],f[b]}

{f[b],f[c]}

{f[c],f[d]}