I don’t have any experience of 知青's life but I had worked with 定量标准42斤 for 3-5 years (some of my co-workers had 46斤). I still remember the days when we tried everyway to find a restaurant selling 议价饭 even with such a 定量标准. So I think what 田牛 said is understandable. The explanations could be:
1. heavy work
2. 没有油水
3. 年轻,长身体
4. indicidual difference