Tentative hypothesis: some people start with the intention of making rules they'd want to follow, and others don't. The first set might find themselves with a rule they don't follow, but the second assuredly will.

This goes beyond the temperamental difference between people who find rules a reassuring way of limiting choices and those who find rules an irritant at best.

How much care do you put into crafting your rules?

This is a valid attempt to deal with conflicting stimuli from the world - to create standards to which you adhere consciously because you don't trust your intuitions to motivate you rationally in the environment with which you must interact. And really, such attention is partially what it means to be conscious/human - to audit your actions 'from the outside' instead of merely reacting. And with today's bizarre and skewed 'food environment', as it were, this becomes VERY necessary, especially for people with a predilection for analyzing their own behavior even in such supposedly mundane (but really fundamental) things as food consumption.

The rules were supposed to approximate her actual tastes, but more rigid and outright made up when she was unsure if she liked something. I don't think it would work if she suddenly decided she disliked peanut butter.