1 backpack doesn't mean 1 item. 1 backpack could be counted as 4 items.
After a wave ends and everything dropped gets pulled to you, it equates to a number (for example 10). In between waves, you can see your current total of Food, Materials, Survivors, and Items. The Survivors (for example 3) and Items (for example 7) when added will equal the number you saw previously (10).
It is thought that Backpacks don't add any additional XP, Food or Materials (Since you can see separate food/material icons drop instead of backpacks for them).
Different colored backpacks do NOT give different types of rewards. Though it is possible that
different coloured backpacks give higher valued rewards. As the brown one appears to drop
items of the greatest value and the gray the least.