of course Gazelle can offer higher prices. They dont have brick and mortar stores nor do they have sales people. Plus Apple won't accept low margins like a company like Gazelle.
Gazelle also does not guarantee the price. You need to ship it to them and 'hope' they appraise the phone the same way you do. As far as the highest price from Gazelle, good luck getting that price from them. Personally I've never heard of anyone getting their phone appraised as 'flawless' from Gazelle. Plus all the trouble of shipping, trouble of depositing the check, trouble of waiting for weeks, ect, ect.
For many people the extra $40 is just not worth it. Most people buying iPhones make more than $40 an hour so its not really saving them anything for wasting their time.