Look at these users:
They gained high ranks of last 4 Div.2 Only contests, maybe few of them are really new users but it seems that most of them are div1 users who register new accounts for participating in div.2 only contests officially (look at their register and last visit time).
I think there is no difference in participating officially or unofficially, this only makes the competition harder for div.2 users for they should compete with div.1 users.
Anyway this is wrong, but high number of div.2 only contests is also a reason!
Please prepare contests for both divisions more often!
UPD Is there anyway to prevent from such multi accounts?
UPD2 Preparing contest for both divisions is the ideal solution! And if it is not possible, as most of the comments say it is a good idea to rate only users that have participated in at least one (or more) contests!