This is what you said: They got you to think that if the price of filet Mignon goes from $30 a pound to $32 a pound ( a 7% increase) and the price of Flank goes from $2.89 a pound to $4 a pound ( a 72% increase) that you will instead by the Filet Mignon.
You seemed to say that BLS assumes that a consumer faced with a 7% increase in filet mignon and a 72% increase in flank steak would choose filet mignon. The filet costs $32; the flank costs $4. If their substitution assumption is that consumers picked the filet mignon, because the price increase was smaller, then the consumer in your example spent an extra $28 in the beefsteak category. You'll have to correct me if I'm not parsing your words in the right way. You seemed to see the same thing above: BLS assumes the consumer substitutes for the lowest percentage price increase.