I suspect a mis-spelling of the name, or, more likely, a change in the the spelling as people became literate. The only Nellis family that I know of prior to say 1840's were mainly found in NY and Ontario plus one in Pittsburgh. The NY and Pittsburgh family is one I know well. I know, that during the later mass migrations, a significant number of Nellis families did migrate. They came from virtually every European country.
I do not think there was a second Nellis family in America in 1790, let alone in Eastern Pennsylvania. Trying to locate the PA family earlier than 1790, I carefully checked Conrad Weiser's followers from NY to PA, there were no Nellis's among them.
Doing a search on Ancestry.com for the 1790 census, I did find the following sounds-likes: George Nulse Nazareth, Northampton, PA 1790 Casper Nulse Towamensink, Northampton, PA 1790
Certainly "Nulse" could have mutated into "Nellis"