They didn't want to have Terry Farrell in such heavy makeup and prosthetics. They felt she was to pretty to have it covered up. So some speculate the Trill in "The Host" was a sub species of the Trill race. As far as i know it that theory is not cannon. the in universe explanation is that there are two different humanoid species used as trill hosts.
The real reason was that the producers were not satisfied with how the ridges looked on Terry Farrell (Jadzia) during early makeup tests prior to the pilot of DS9Makeup change.