The figures are based on the tips that were reported by drivers. Drivers have an incentive to under-report cash tips (income that's not reported can't be taxed)

The figures are based on the tips that were reported by drivers. Drivers have an incentive to under-report cash tips (income that's not reported can't be taxed), but not credit-card tips (which can easily be audited). It's possible that drivers aren't earning more, they're just reporting more.