Usually it's because the 'bars are lower relative to the saddle height on a roadbike, which tilts the upper body further forward and puts a little more weight onto the hands/arms.
If the bike's been properly fitted, the reach and saddle/bar drop should have been optimised for your build and reach, and you will get used to it. sore/numb hands are often due to the "death grip" that riders new to road bars seem to think is essential due to the faster/twitchier steering of a road bike.
I'd personally prescribe some decent well fitting mits (ok, given the time of year - gloves) and if that doesn't help - perhaps try something like the Specialized "bar phat" gel insterts that go under the tape. I've one bike that's got oversize ally bars and after 60-70 miles it would get a bit painful, so I added a set of the bar phat gels, and it seemed to cure it. The Dolan doesn't suffer from that, the carbon bars seem to smooth things out much better.
