general expression formatted into a string
For a while now, I've been puzzling over why webMathematica inserts characters around output under some conditions. Now I have my answer.
If a general expression is evaluated, webMathematica formats the output as a string with HTML tags.
To avoid the extra tags, use ToString, around the expression, in your mathlet.
&myResult=<%Mathlet
ToString[ Sin[$$num]]
%>