HAProxy domain name in map_dom

I have a haproxy config using maps.

HAProxy config file has the below line:

use_backend %[req.hdr(host),lower,map_dom(/etc/haproxy/domain2backend.map)]

And in the domain2backend.map, i have the below entries:

    dp.stg.corp.mydom.com dp_10293
    dp.admin.stg.corp.mydom.com dp_10345

Now when i access dp.admin.stg.corp.mydom.com/index.html it is directing me to backend dp_10293 . However using a simple full string match of map(/etc/haproxy/domain2backend.map) solves the problem and it directs me to proper backend dp_10345

So how is map_dom comparing the domains and how is it directing request meant for dp.admin.stg.corp.mydom.com to backend of dp.stg.corp.mydom.com