The oil ring gets gummed up and then stops doing its job, allowing oil to stay on the cylinder walls and thus be burned. The rings we put in are of a new design to prevent becoming sludged up, or so we are told. The updated oil specs are supposed to prevent sludging as well. That's also why the interval does not change to 10k miles.
I've done many of these. The oil ring is always full of sludge. If the rings didn't take care of it, I suspect either the cylinder wall became scored (which should have been checked for during the job) or perhaps there was a mistake made while replacing the rings (improper orientation or inadvertently cracking a ring and not noticing it.