\" in $(( )) vs $[ ]

[Mikael Magnusson <mikachu@xxxxxxxxx>]

% echo $(( ##\' ))
39
% echo $[ ##\" ]
34
% echo $(( ##\" ))
<cmdsubst:~>%

and tangentially, why does the above also work with only one #?
% a=b
% echo $[ #a ]
98
% echo $[ ##a ]
97
% echo $[ #\' ]
39
% echo $[ #\" ]
34

-- 
Mikael Magnusson