There is a percentage displayed in the Careers profile.
Does anybody know what the basis of this is?
The basis is your total tag score (which consists of both question and answers, non CW) as can be seen in your profile:
Which is then compared against the scores of all other users who took part in this tag.
As posted in the other answer you can see the top 20 users of the tag and to get full list you can use the Data Explorer. (will write something up if I'll have time :))
It means that you're in the top 20% of all users in the google-maps-api-3 tag.
https://stackoverflow.com/search?q=user:884521+[google-maps-api-3]
here's a better link