Your method doesn't control for shrinkflation[0] if only looking at your debit card statements.

[0] https://www.mouseprint.org/category/downsiz/

Edit:

Humans aren't robots folks, if there's consistently some excess food being disposed of it could simply mean there's slightly less going into the wastebin.

And regarding TP, I don't know about you, but I tend to be wasteful with TP when the roll is fresh and increasingly frugal as the roll approaches empty.

This is all highly subjective but the fact is people tend to be inconsistent and wasteful.

You need to track normalized unit costs for such data to be reliable. Package sizes are constantly being manipulated, period.