There are two possible reasons. The internal voltage regulator has max output somewhere between 300mA and 500mA (can not be determined exactly, because the number on the chip does not identify it), the same goes for the regulator that supplies the USB port.
If you add up all absolute maximum ratings , you can get up to 1A, but the actual consumtion is nowhere near that.
Computer USB ports supply by default 100mA USB 1.0, 2.0 , USB 3.0 150mA, but depends on the computer. Higher current draw is only enabled if there is a communication through the port and the device negotiates a high-power mode. Bu then again, not all computers follow standards.
My 703N works just fine from a mac laptop, whithout an additional USB modem.