What PT is saying is that you've drawn the plates closer together than the OP says they have to be.
Or, that the OP allows the table to be bigger than you've drawn it.
That is, if you want to prove that a certain number of plates is enough to cover a table [any table] consistent with the OP,
then you have to deal with the most difficult case that the OP permits.
Putting it another way, if N plates cover the table you drew, but won't cover a bigger table that is still consistent with the OP,
then you haven't proved that N guarantees coverage. That is, a counter-example [bigger table] exists for which N fails.
But if N suffices for the worst-case [largest possible] table, then you've proved that N will succeed for any table.
The OP has the word ensure in it.