Retrieving Mentions for Other Users - Logic



I am interested in searching for user mentions, specifically for other users. I understand that this is possible via the search API, by providing @username as an operator.

However, when executing such a query through the regular web based search interface, I see that in addition to those tweets which explicitly include the @username within them, other tweets are also included. For example, when searching for @BarackObama, in addition to tweets including this mention explicitly such as “New @BarackObama energy secretary wants more nuclear power plants. How do San Onofre, Keystone foes feel about that?” it also includes tweets including his name such as “The internet is awesome. Barack Obama spliced up to sing Get Lucky.”.

Could someone explain the logic behind this, since I imagine that the API works in a similar manner? I would have expected that the search should return only those tweets with @username mentioned.

Many Thanks.