When you execute a query that uses your user ID, it is prefilled automatically if you are logged into the Data Explorer.
If, after executing (because before that it isn't really possible for some reason), you select a different site's icon, the user ID is carried over. This happens with all other parameters within a parameterized query, too.
The only problem is: Now, it's wrong. While carrying over other parameters for convenience makes sense, the user with the same ID will be a completely different one between different sites. In fact, it's a bit annoying when trying to do ego-queries on several of your accounts, because you have to keep removing the UserId parameter from the URL to get it prefilled.
(The only exception where user IDs match is between a given site and its meta site, barring MSO.)