Geologists tend to walk along the surface of the earth from which faults project downward. So, dip direction is the downward direction.
Seismologists live down about 15 km depth (in a big air-conditioned control room) and think of faults extending upwards. I work with an earth scientist who published papers showing little upper hemisphere arcs to represent faults...and it really took some mental gymnastics for me to read these...plus, there might be many 1000s of these on one plot.
Chris