They probably round it down on the wiki so that people assume they can use 260k and don’t get in trouble when they actually use 260,010, for example. For example if you’d store the strings in pieces in a table, there’d be some overhead apart from the strings in the table.
The limit is imposed by the software that they use, not by the amount of servers or the amount of storage space (storage is super super cheap, not an issue usually). They (most likely) store their game data collections in database software called MongoDB, since it enforces a limit on max number of splits that matches the limit that we have:
So if they want to support larger sizes for datastores, they either need to split data over multiple chunks, choose a different database for the back-end of game data, or I guess MongoDB could get an update that supports larger chunk sizes in the future, but the latter seems unlikely.