Khan says he is on Ceti Alpha 5, that Ceti Alpha 6 exploded and shifted the orbit of Ceti Alpha 5. Chekov and Tyrell think they are on Ceti Alpha 6. Despite the explosion, Ceti Alpha 5 would still be the 5th planet out from the star Ceti Alpha. So the Reliant should have realized this planet was still Ceti Alpha 5, assuming they can count to 5 and on to 6.