Current food production is showing at 10, and it would be a little rediculous if I have to plant extra crops once I have 11 settlers. Technically there's already enough crops planted to easily feed +20 people. I'm on XboxOne.
So there will be a few crops that technically don't assign?
Don't worry, carrots are cheap, so is dirt (as the expression goes).
Try removing the crops that your having issues assigning.
I'm thinking you are trying to do the assignments early on, when there are just the three Abernathys. In which case they were already assigned to the crops. I'm thinking that you don't get the option to Assign a settler to a task that he is already performing.