Canning machines are great and everything, but some food fills more than one can! Usually they fill the same amount of cans as full hunger bar points, but what happens if something fills half? Check canning points here.
The can yield for a single piece of food is ceil(food_points/2). That is, .5s on the hunger bar are rounded to 1s in cans.
You can also can rotten flesh (2 cans) and spider eyes (1 can). This negates the negative effects of consuming these items.