Thank you Kuba!  This makes sense.  
I noticed that if I used "/" instead of "Control+/" it works but does not convert to traditional form in either the input or output.  Is there a way to fix this so it converts?  The code below coverts to traditional form... but does not use boxes.  Is there a way to convert with boxes?  
Manipulate[Row[{a, "+", b, "=", Simplify[a + b]}], 
 Row[{Control[{{a, 1/2, ""}}], Control[{{b, 1/2, ""}}]}]]