system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some ships’ decks numbered/named from the bottom of the ship, and others are named/numbered from the top?
gpt
Former Squid here, as far as I've ever known it was just sort of the convention. At some point in the past it was convenient to start numbering, and if you chose to start from the top or bottom was fairly arbitrary excepting perhaps in cases where it is easier or more clear to differentiate between certain numbers depending on the language. But one specific designer or organization is likely to stick to one method because that leaves less room for confusion. 
There's even a third option. Numbers that start in between top and bottom! Our (US) aircraft carriers do that. You start with the flight deck, and the numbers increase as you go down. But if you go up, vertically, now you're on levels, not decks, and those numbers also count up numerically. The decks are just called like "3rd deck" while levels are "the O-3" (that's a dash not a negative). It helps prevent having to talk about decks or levels in the teens or beyond. Given how much we love to shorten names and messages, ease of use seems to be the culprit.