It's not entirely the same power supply.
The differences are in the 3+ which has 1 Ohm added to the outputs (behind the caps after the regs) and a set of extra reservoir caps near the output devices.
Differences in hum could be caused by idle current differences and/or ground/wire routing differences as well as PSRR of the circuits itself.
From my observation of the pcb with only 4 of the 1 ohm build-out resistors per channel, it looks as though only one of the dual OPA1688 amps in each package is used. This would ease thermal stress.
An impressive little part for ~$0.50 in quantity!