Why is it not recommended to mill deer?

I made this table for wood to food efficiency:

Farm: 175 food, 2.9 food per wood, 0.33 food per second

Horse collar farm: 250 food, 4.1 F/W, 0.33 F/s

Heavy plow farm: 375 food, 6.25 F/W, 0.38 F/s (with Wheelbarrow)

Milled deer (4x): 498 food after decay, 8.3 F/W, 0.41 F/s

Crop rotation farm: 550 food, 9.1 F/W, 0.41 F/s (with Hand Cart).

It appears milled deer are almost as efficient as a farm with Crop Rotation and more efficient than a farm with Heavy Plow and Wheebarrow. However, deer are available in Dark Age. These numbers make me wonder why it is not recommended to mill them instead of building farms.