It just means that there are probably 2 2-toppers available , but other groups of 4 have made ADR's for the 4-topper tables available. Since most 2-toppers are together - Disney advises your party may not be at the same table. Depending a ton of things more than likely your party will be seated together if at all possible. Disney is just saying it is a possibility and if you don't want to do that or take that chance you can look for a different place that has availability for your party size.
More than likely it won't be an issue, but Disney is just saying that they do have the space - just not in one table - they're letting you know what could happen. They're giving you the option of what you want to do. Some people have their heart set on a particular place and are willing to do whatever to eat there. If the place is booked up, its already full - Disney is doing its best to accommodate your party if that's really where you want to eat.
When you call Disney dining or make the reservation online - they will tell you that upfront as your making the reservation. Its not standard. We have multiple reservations for our party of 4-6 people over the summer and none of them have that notation, but we booked 180 days out.