Queries can take some time to refresh once they have been tested once due to DNS caching. So if you have just updated your reverse DNS record it might take some time for it to refresh.
"Why don't all the addresses have a reverse DNS record, and rather have an IP address?" That is a very good question, the answer is that the host probably doesn't have one. If you don't want to see such records turn simple to 'yes'.
"How do I get a reverse DNS?" Reverse DNS is done by your provider. To get your address setup with one simply get in contact with them and indicate what address you want your IP to point to.
"What is reverse DNS?" DNS is a system where bulkrdns.com gets changed into an ip address like "22.214.171.124". Reverse DNS is the opposite where "126.96.36.199" is changed back into bulkrdns.com. This is used for many things, but not limited to security, email systems, identification of systems, and much more.
"Do I have to do bulk lookups, or can I just do one?" Yes you can just do one, simply don't put anything in the "address to" field.