Follower Count doesn't match Email Count – why?

I have a new blog self-hosted running Twenty Eleven and using Jetpack.

My Askimet site stats say I have 22 followers. Below that it states I have 18 email followers. My questions are:

1) who are the other 4 followers (i.e. 22 minus 18)? Are they following me in a different manner from email?
2) If not an error, is there any way to determine who the additional followers are?

Thanks so much,

Jen