Prescription Benzoyl Peroxide vs Over The Counter BP

What's the difference? My derm prescribed "5% Benzoyl Peroxide Aqueous Solution".

What is an aqueous solution? Is it made differently than OTC stuff? Any significant advantages? I find it hard to believe that prescription companies could make a profit off something that you can buy OTC for ~$3 in Wal-mart. There must be some difference.

So what separates Benzac AC, Brevoxyl, Benzagel, etc etc from say Clearasil or Oxy?

Thanks!