Villagers use inconsistent amounts of food when breeding

Steps to reproduce

1. Make two small chambers with 2-block high walls.
2. Place 4 beds in each chamber.
3. Spawn 2 villagers in each chamber.
4. In chamber 1, give one villager 24 carrots and the other villager 0 carrots.
5. In chamber 2, give each villager 12 carrots.
6. Stand back and wait a while.

Expected result

Villagers in each chamber produce the same number of babies. (I am not sure if it should be 1 or 2.)

Observed result

The villagers in chamber 1 produce 2 babies. The villagers in chamber 2 produce 1 baby.