I think it would be more accurate to say that the reruns *were* shown under the title "Ponderosa." Back in the era when "Bonanza" was on the air, it was common practice that if a show had been on the air long enough to enter rerun syndication, but was still on the network with fresh episodes, they would give the syndicated reruns some sort of different title to distinguish them from the current-season episodes. But once the original series went off the air, and there was no chance of confusion, the reruns usually reverted to the more-familiar original title. The last show I can remember doing an alternate title was "Happy Days," which used "Happy Days Again" as its title when it was first syndicated. Nowadays, I think people are familiar enough to understand the difference between first-run network episodes and syndicated reruns, and the reruns are more likely to be on a completely different platform anyway -- a cable channel, or a secondary broadcast channel, or a streaming service. Anyway, I haven't seen "Bonanza" reruns under any title other than "Bonanza" in decades.