You are reading the results incorrectly. Flight 1368 is not showing as pricing as $1446 or $1534. Those prices are roundtrip prices, not one-way prices. What it is showing you is the cost of flight 1368 outbound plus the cheapest possible return flight option. Adding additional airports to the search criteria will potentially include cheaper return flight options to the other airports in the roundtrip price. You are being charged the exact same amount for the outbound flight 1368 in both cases. Including FLL in the search criteria causes it to include a LAS-CLT-FLL return option in the roundtrip pricing which results in the $1446 roundtrip price. Excluding FLL leaves only MIA return options were the lowest possible roundtrip price is $1534.