If you're using the DNN Friendly Url provider, then the rules that your portal is following are in the friendlyurls.config file, located in the root directory of your site.

The behaviour you are noticing is the 'fall back' behaviour of the standard Url Rewriting scheme.   If you supply a tabid that doesn't match the domain name that you supply, then by default you will be redirected back to the first portal alias of the host portal.   Because if you mess with the 'tabid' parameter, this means that the regex rule for the tabid's won't match anymore, and hence you won't get a tab match, and hence you'll get redirected to portal Alias 0 of portal 0.

Incidentally, you probably found that changing the 'home' value of the url doesn't make any difference.  That's because the regex rule matches */tabid/dd/* - it's only the tabid/xx/ part that needs to match to get a valid tabid and thus show the correct tab in the correct portal.