My guess would be that it just depends on whether or not the server your pulling the install from is on the same subnet or not.
In the few experiences I've had installing over the network, they did not go well. Meaning the install took much much longer over the network (up to 45 minutes) while copying locally took about 5 minutes to install.
Even installing from a server on the same subnet took longer than installing locally.
I don't know exactly why, open a support case with Symantec to see what assistance they can provide, but as already mentioned above it is best to copy locally then run the install.