It is a disclaimer. Easier for them to have it on every reservation. If they end up with more large groups than they have large tables, they may place you are 2 physically separate tables. You may end up at 2 tables that are next to each other, but may be separated by an aisle. That type of situation. They will do their best to ensure that your entire party is seated together, but want to make sure that you know your party may be at 2 tables.
To the PP asking about her family of 5, they won't split your group up if it is 2 parents with 3 kids. Not gonna happen. Your wait may be a touch longer til they find a table that can accommodate you, but they won't split you up.