Because the hosts I monitor change with some regularity and are added and removed on a frequent basis.
Actually, no. I thought so too at first but there are no errors and the hosts that were exhibiting the previously described behavior had the same configuration attributes as the localhost, which does not act in the same manner.
I did end up figuring it out though and in reality, the answer is quite a bit simpler. The time indicated in the host status field is the time relative to the host's last status change. When a host is newly added it's status initially has a value of PENDING, until the first host check is performed. From PENDING it will change state to either UP or DOWN. If the host goes from PENDING to UP status, the duration begins counting up, but Nagios does not consider this a status change as far as the retention data is concerned, and if a reload is issued before the host changes state again, the counter will zero-out. However, once the host goes down (changes state) for the first time, counter values will hold over reloads. The same is not true though if a newly added host goes from PENDING to an initial state of DOWN. In this case, the counter values are held over reloads.
Nagios identifies hosts whose counters zero-out on reload due to never having changed state (or perhaps more accurately, whose initial state has not been time-stamped to the retention file) by putting a plus (+) to the right of the elapsed time value. This plus will remain until the host changes state for the first time, placing a value in the retention file. The presenance of this indicator leads me to believe the logic described above is done by design and not by error. What I haven't determined yet is if there's a setting to change this and treat the counters for a new host that goes from PENDING to UP the same as one that goes from PENDING to DOWN as it's initial state.