Well I am not too sure but I think they are experimenting rather than offering a 48V setup. And it even be a consequence of reading these pages over the last 5 years.
My own chair uses a 45V setup, (47.X Volts charge, 43V running voltage etc) for 16mph. (8.5mph motors on double the voltage)
And the reason for them trying going to "higher voltage" is the same as the reason I did it.
Lithium doesn't suffer from voltage sag (peukert) in the same way that lead does. So a smaller Ah but higher voltage battery becomes workable with the same peak Amp draw. At least quality high rate cells don't if the capacity is maximised too.
So instead of making the gearing taller - the usual way to increase speed - which reduces torque/increases battery Amp drain in direct proportion. You can just increase voltage and double the speed without any range, torque, downsides. A huge efficiency win of about 100% that seems impossible on the face of it! But its really true. It works because the controller is actually an electrical "gearbox". If you do the maths it behaves exactly as it should.
My chair is here:
http://www.wheelchairdriver.com/BM-MK3- ... rchair.htm