man(1) lookup: how does it work?

[Brian Holtz]

How does man(1) know which man page to read when the topic is
not the name of the man page's file?  For instance,
"man hashstat" reads the csh_builtins man page automagically.
Now, "hashstat" is in the whatis database file as one topic among
many on a line that starts with "csh_builtins"; a cursory glance
through the whatis file seems to show that to be a trend -- the
first topic on multi-topic lines is often the same name as the
man file for that set of topics.  Is this just a coincidence?