I also get this for the first few miles when the car is cold. I blame the temperature and also the torque converter.
In 1st and 2nd gear sometimes I can go up 1k revs and the car won't move in relation to the revs. It's a very odd feeling and I dislike it. I end up compensating and taking things even slower.
However, as soon as I reach third gear the torque converter settles and no slippage..