I've very rarely seen locomotives whose eccentrics (or whatever mechanism drives the valve gear) aren't mounted on the same axle as the main rod. I always assumed that was to avoid a possible 'accretion of error' situation potentially caused by brasses needing adjustment, loose driver boxes, that sort of thing.
As Benjamin points out, it's quite possible they're following the original Porter practice, and for the same reasons. I've never had the chance to examine one of those wonderful little 8-tonners in person and find out.