The reason for this can either be the rabbit has never been infected, the rabbit was infected in the past and has cleared the infection and the antibodies are no longer produced, or the rabbit is currently infected with Pasteurella, but because of a poor immune system, has not produced antibodies. The Pasteurella antibody test is useless if you are using it to see if your rabbit is infected. The Pasteurella antibody test is useless as a screen for disease. This test has nothing to do with disease status, just immune status. Look at the above paragraph.
Infected rabbits can have either a positive or negative titer.
Non-infected rabbits can have either a positive or negative titer. I am not sure there is any reason, in most pet rabbits, to do this test. The results can be misleading if they are misinterpreted. In a lab animal facility this test may be important, but not in a pet rabbit situation. If a reliable Pasteurella antigen test becomes widely commerically available, this may be more important to pet rabbits.