The men that returned from battle received double rations, why is this?

The men received double-rations because half of them had been killed at the front.

From the text:

Fourteen days ago we had to go up and relieve the front line. It was fairly quiet on our sector, so the quartermaster who remained in the rear had requisitioned the usual quantity of rations and provided for the full company of one hundred and fifty men. But on the last day an astonishing number of English heavies opened up on us with high-explosive, drumming ceaselessly on our position, so that we suffered severely and came back only eighty strong.