This is because your weight is in relation to earth. To keep ourselves off the ground, or earth, in a hammock we commonly use two suspension lines so our total weight is being shared between the two lines.
For a 250 lbs person the weight carried by each end of the suspension would be 125 lbs.
If you were to rig a loosely termed hammock from a square tarp tied between 4 trees the weight carried by each suspension line for our 250 lbs load would be 62.5 lbs per line.
I hope this helps.