Mine alternates wash arms too on all cycles, but the Heavy Cycle seems to concentrate the lower wash arm predominantly once the main wash phase begins.
I'm a bit late to the party, but my Kenmore dishwasher model#630.13902011 also starts with the drain pump running, then it will do the main wash pump to check for cavitation. I'll have to test different washloads in my apartment.
I hate washing dishes by hand, so I'm not gonna complain.