[web2py] concatenating HTML helpers

[apple]

If I try to concatenate a TD and an XML using += then I end up with a
string. Why is that?

e.g.

var1=TD("aaaa")
var2=XML("bbbb")
print(type(var1))
print(type(var2))
var1+=var1
print(type(var1))

===> TD, XML, string