If you are using only one ink in your profile any blocking of the nozzle could cause big difference in density.
I have worked with many Epson printers in the *800/880 line and they all printed with almost identical transmission density in profiles when using only the K inks, or with K and Y. By almost identical I mean within +/- log 0.05. The only thing I know that could cause a big difference in density would be nozzle blocking, or some difference in the profile or profile settings.