Why the starter pokeballs were empty in the 1st episode

When ash is late to get a starter he tries the pokeballs and the are empty with oak stating that the starter was given to a trainer that was on time.

But if they were taken then shouldn’t there be no pokeball as it’s with the trainer. So why set up empty pokeballs ?

From the celebi movie Sam oak knows that ash and pikachu have a bond and it’s his starter but he never knew that ash was late to get his starter.

Oak set up the empty pokeballs to say that he was missed getting any of the starter trio and the pikachu is the only option available even if he did show up on time