I'm no expert, but the difference is to do with the soil profile.
Ashburton, with very limited resources, on the whole produce a decent track. They have strong nor'westers to deal with and light, free draining soil which dries out quickly.
The ground down the back straight yesterday was pretty dry but the home straight looked level with a reasonable sole of grass.
Upgraded from 4 to 3 after consultation with jockeys.
Riccarton has permanent track staff but the compacted nature of the soil makes for an unforgiving surface with no yield for the horses. As well there is an unfortunate history of track ratings not being overly accurate and maintenance not pleasing many.