Well, if you bought a version of 13 that had its price reduced because it was an upgrade-only product, then, yes, it would expect to see a qualifying version already installed. Now, I've never done that with PE, but if it's a version that can install only as an upgrade, you must make sure that PE 7 - PE 12 is a qualifying upgrade path. For example, it's conceivable that PE 7 is too old to qualify for an upgrade to 13.
If it is, but the installer didn't see it, other installation programs might stop and give you a chance to enter the older version's product code. Did you see anything like that before the installer quit?