It would appear that the account's "Last activity time" is not updated for every hit. I've been active on SO all morning, and in the last five minutes have voted on some posts as well as posted an answer. However, my account profile says:
"Last activity: 2 hours ago from <IP address>"
Is there a certain amount of time that needs to elapse between hits before the activity time is updated again?
This issue could result in Enthusiast badges not being awarded when they should.