Operator implicit conversion difference

[ixid via Digitalmars-d-learn]

This may have been overlooked in my other thread so I wanted to 
ask again:

This seems very inconsistent, does a += b not lower to a = a + b? 
I guess not based on the below:

     ushort a = ushort.max, b = ushort.max;


     a += b; // Compiles fine
     a = a + b; // Error: cannot implicitly convert expression 
(cast(int)a + cast(int)b) of type int to ushort