Bgomes is probably right on this one. I'd just like to add though that I've found a helpful way to avoid seeing a high "blocked from mailings" number is creating a global smart list that factors out any addresses that would be ineligible for mailings in the first place. These filters include:
Opt Out: False
Email address is not blank
Black list: False
Email Invalid: False
Marketing Suspended: False
I then reference this smart list (I call it "Eligible for mailings") in every outbound email campaign i send. It helps me get a much more accurate prediction of send size, and it reduces the head scratching.