[Exim] Router condition

I'm intrigued why

condition = ${if exists {$home/.bounceptr} {${lookup {$sender_host_name} \
nwildlsearch{$home/.bounceptr}{yes}{no}}} }

would cause a remote IP lookup ($sender_host_name) when the first part of
the condition (if exists) fails - ie. there is no $home/.bounceptr.

The condition doesn't fail or anything wrong, but a lookup is being
forced.