Why is the ABI for structs different between Linux and Windows?

[Don]

 From the ABI page, there are 2 ABI differences between Windows and 
Linux. One is the amount of padding after a 'real'. That's 
understandable, because it's imposed by the OS. But the second 
difference relates to structs, and is non-obvious:

# For Windows, 1, 2 and 4 byte structs are returned in EAX.
# For Windows, 8 byte structs are returned in EDX,EAX, where EDX gets 
the most significant half.
# For other struct sizes, and for all structs on Linux, the return value 
is stored through a hidden pointer passed as an argument to the function.

Why the difference between Windows & Linux? Is there a good technical 
reason for it? Or is it a quirk that we might be able to get rid of 
eventually?

It particularly looks like a quirk because the section on parameters states:

#The last parameter is passed in EAX rather than being pushed on the 
stack if the following conditions are met:

     * It fits in EAX.
     * It is not a 3 byte struct.

which sounds as if 1,2 and 4 byte structs are placed in EAX for 
parameters, but not return values.

If it is a quirk that may be removed, I suggest an extra sentence:

# For other struct sizes, and for all structs on Linux, the return value 
is stored through a hidden pointer passed as an argument to the 
function. In the future, 1, 2, 4, and 8 byte structs on Linux may be 
returned using the same calling convention as for Windows.