According to the Event Guest List columns documentation, the Ticket Price is the dollar amount per ticket and Total Purchase Amount is the total dollar amount the ticket purchaser paid for the transaction. Why could these amounts be different?
The Ticket Price is showing the current price of the ticket. The purchase amount is showing what they paid at that time. It's possible they purchased multiple tickets in a single transaction which is evident by the number of guests. If the math doesn't add up that way, then that means the ticket price was set to a different price at some point in the past. If you download the Event Guest List report in to Excel and sort by Registration Date, you'll see when the purchase amounts started changing, which should indicate when the ticket price was changed.