I'm fairly sure it's because the `:limit` constraint has gone away. It looks like AR places all `:limit` constraints in a column, then all the `:default` constraints in the next column and finally all the `:null` constraints. By removing the only `:limit` constraint in that table, we've triggered AR to shift both the `:default` and `:null` constraints one column to the left.

As such, I believe we can guarantee that it'll behave consistently across machines.