Here is the short and sweet of it:
"It's simply a matter of how you count. You can count each period as fourteen generations first by extending from Abraham to David; secondly, by extending from David to the deportation to Babylon in the time of Jechonias; and thirdly, by extending from Jechonias to Christ. Matthew was aware that he was being inclusive or doubling up on one generation in each case."
From the Forerunners Weblog