In a nutshell, Blogger have (as we suspected) been tinkering with the Followers service.
Quote from a post on 'The Real Blogger Status' dated Monday December 21st:
During the past weekend, Blogger started the process of simplifying Following. This involves removing non Google accounts and profiles from Following.
As part of this plan, starting the week of January 11, we’ll remove the ability for people with Twitter, Yahoo, Orkut or other OpenId providers to sign in to Google Friend Connect and follow blogs. At the same time, we’ll remove non-Google Account profiles so you may see a decrease in your blog follower count.
This is the link to the blog, explaining in more detail
Hope this enlightens those of you who've been wondering!