XRumer users now (actually they've been using it a while) use something called xblack.txt, this is basically a list of know reporting sites. What this allows them to do is avoid detection for longer, so the public API's become less affective at catching spam bots (Indecently, if your site reports lots of spam bots, you'll get attacked by bots less because of this)
APIs will become less affective overtime, but they're still fairly good for now
To be honest, I'm not a big fan of using multiple API's to stop bot spam, 1 good one is often suffice (StopForumSpam/StopBotters seem to catch the most for XenForo), the more you add, the slower it becomes for the user to register.
But multiple mechanism (that never affect humans), that is useful (for as long as the bots don't bypass them)
Hotmail/Gmal/mail.ru... it's all the same, botters will pick one favourite for a while, and then change
Sometimes certain email providers make it easier to automate email account creating (Yahoo was a favourite a while back for this), but you'll see cycles of patterns as the botters (sometimes just a handful or actual bot users) jump from one preference to another.
What you might be seeing is.. that this pattern is from one (or a few) botters that are using xblack.txt, and they have recently automated email accounts against hotmail.. that's it. The pattern probably wont persist for more than a few weeks (if that). I don't believe Hotmail is currently a botters preference for email creation, they are all as easy as each other to automate.