Fractionation is a physical process. Here is the
Wiki.
Essentially, given a container with a mix of different boiling-point compounds, and an escape route, the lower-boiling point compounds will preferentially exit the container, thus
fractionating the remainder to a less-volatile residue. The first burn is like cherry-picking, and subsequent burns are, in insurance terms, "Adverse Selection", which is why, as you have advocated, the inverted canister is the correct solution. QED.
