* "Philosopher's Stone" became "Sorcerer's Stone": The publisher, Scholastic, felt "Sorcerer's Stone" sounded more appealing to American children. They thought "philosopher" might be too complex or confusing for the target audience.
* "Order of the Phoenix" remained the same: This title wasn't altered, as it was deemed strong and evocative enough for both markets.
Essentially, it was a strategic move to make the books more commercially successful in the US. It's worth noting that this is common practice in publishing, where titles are often adjusted for different regions to resonate better with local audiences.
While some argue that the original titles are more accurate and nuanced, the American titles have become widely accepted and are considered synonymous with the books.