I think part of that is weather at the start. Hiking in early March in Georgia is different from hiking in southern California desert in late April.
The other part may be resupply and water -- if I know I have to hump a gallon or more of water, and carry a lot of food, I'm more likely to keep my pack weight down to start with.
Finally, overall weather and rain conditions are different, most of the way. The limited hiking I've done out West was *very* different than AT Hiking in terms of rain, in both duration and amount.