Sorry guys - nothing as technical as wing sweep!
Numbers in the 400 series had been used up, the 500 and 600 series were being reserved for missiles, pilotless aircraft and gas turbines, hence new aircraft types would start afresh in the 700 series.
The 707 should really have been called the 700 but Boeing felt that 7 was a lucky number and having two 7s in the designation would mean twice the luck. The head of Boeing public relations, Carl Cleveland, agreed that Model 707 sounded better and the rest they say is history...
Hope that clears up the mystery! Wonder if the proposed Sonic Cruiser will be given an all-new designation?