When forming a caravan, food rot time estimation changes in strange ways.
First of all, it changes based on who you're putting in caravan.
Selecting Sharr, food rots in 0.8 days
(http://i.imgur.com/M2RF97f.png)
Selecting Abram, food rots in 1.6 days
(http://i.imgur.com/apB06so.png)
Further, adding food on items tab seem to not change rot times at all
At first I thought it was considering food that colonist are bringing on their own ("food reserve" shown on gear tab) but Abram and Sharr both have simple meal, so I expected time to rot to be equal.
The rot time for the simple meals they are carrying as their food reserve depends upon how long they've been carrying them around unrefrigerated, so it's expected that they would have different rot times.
Zhentar is correct, so it's not a bug. Thanks for reporting anyway.