Plugging in MiFi hotspot to USB significantly faster than on-board modems?

We experienced significantly faster speeds plugging a Verizon MiFi 6620L into the USB port than we did simply putting a SIM into the SIM card slots to use the on-board modem. Is this a common result? It seems very counterintuitive.

@boringITcrap, what product are we talking about here?

It’s the Pepwave Max HD2.

Is it the MAX-HD2-LTE-US-T?