Contract campaign : This was working based on probability algorithm . For example if you set this campaign for , 90 impression and in the time for 1 month . then this campaign daily get views near to 30 impressions . So this was deliver based on probability because this need to deliver up to next 30 days. So only you have seen some times empty banners .
Remnant campaign : This don't have any limitation on probability. So if you set 900 impressions then this should be deliver all impression and this will not keep backup impression fro remaining days.
Contract Exclusive : This was more priority campaign . If zone have more than all the type of campaigns, then that zone first deliver all the impression from this campaign,once this campaign stopped or else completed the finished count then only campaign will be deliver in publisher page .
If you still need any clarification let me know here.