I can show you where the numbers come from. Take a look at table 11-4
Those are the typical loot values for encounters of CR equal to your own level. In a home campaign it would take a party of 4 PCs 13 of those encounters to level up.
So if you take the loot per CR numbers for both levels (for example, 460 and 775), multiply by 13 encounters, divide by 4 shares of loot, and then divide among 6 scenarios, you get pretty close to the amount of credits on your chronicle sheet.