With the Orbital you only want to use gentle agitation, it's not like the continuous agitation of rotary processors. If you over agitate you get uneven processing and the edges get more development, it's worst with the motor base.
This is interesting as I've never found this to be the case when using the orbital on the motor base. I've tended to use Rodinal and Hc110 in the past, and have always used 150ml or more to ensure that there is sufficient developing agent in place, so maybe that reduces any tendency to splash with lesser quantities? Washing is done with the lid off, so I'm familiar with what the quantities look like, but realise that doesn't replicate what the fins on the lid do during processing. The photo below was done with HC110 and the motor base.Yes, using the Paterson motor base for film gives extremely uneven development as the liquid is accelerated around the corners.
Derelict cottage near Garve, Scotland by Northsnapper, on Flickr
Barevan Church tree