I believe the Google Now Launcher for non Nexus phones defaults to 4x4 and can't be changed. Its different on Nexus devices because it's the stock launcher and the grid is set in the build.prop file. It's one of the primary reasons I didn't use the launcher beyond a quick trial and went back to Nova.
I put Google Now Launcher on a S6 Edge Plus in a T-Mobile store and it had the 5x5 layout for it but I've heard the Moto X Pure which has the same size screen defaults to 4x4. Not sure why this is but I hope Google fixes it.