First of all, the term longbow encompasses many variations of bow styles, which technically fall into that category. Longbows like ASL, English LBs, R/D LBs and handle forward LBs like the Shrew, Java Man..., and they're not all created equal when it comes to forgeability.
That said, I will venture a guess at why some LB's may be more forgiving than other bows. My guess would be that LBs with a higher brace height, somewhere in the 8"-9" range, are more forgiving because the power stroke is shorter, and the arrow is on the string less time to be affected by shooting errors.
On a side note, I have always read, or have been told, that recurves are more forgiving than longbows. That is why many archers have trouble shooting LBs, myself included.I'm sure we will get some varying options on that here though.