A lot of Linux systems (most as far as I know but could be wrong) actually ship vim but in a very stripped down build and set up to run in vi compatibility mode

A lot of Linux systems (most as far as I know but could be wrong) actually ship vim but in a very stripped down build and set up to run in vi compatibility mode by default, e.g. vim.tiny on Debian and Ubuntu. BSD systems often use nvi, which is quite close to the original vi, and can feel very weird if you are used to vim.