Question about The Decline and Fall of the Roman Empire

[RCA]

According to Wikipedia, this is a 6 volume series written by Gibbon. My question is why do I see so many 7 and 8 volume sets for sale?

 

[Truth Warrior]

Maybe just so they can charge for more for eight books than for six.

 

[RCA]

Maybe just so they can charge for more for eight books than for six.

Probably.