I noticed in the Bible Sense Lexicon that there are two different sense entries for the animal sacrificed for the Passover: 1. Passover Lamb 2. Passover Sacrifice. I see one is being a clean and the other possibly being unclean. 

I understand that there is a Greek entry and a Hebrew entry, but why would that require two different entries? You can search the entire Bible for a single sense, even though many sense entries contain both Hebrew and Greek words. Is there another distinction being made in this context regarding the sacrificed animals for Passover? I see them as the same thing. Given that the animal is supposed to be a lamb according to Exodus 12, why are the Hebrew and Greek entries different? It would make sense for someone to start with sheep and work their way down to Paschal lamb. Starting with domesticated animals, one would anticipate working their way down