Ok, there are a few reasons why it comes out to be more...
1) Didn't get a detailed quote where you input the box dimensions and the weight, not just the weight. (FedEx charges "dimensional weight" = average weight for a box that size), not the actual weight. I think a 6x6x6 box costs the same for 1lb to 3lbs because they charge you the 3lb weight. Always choose "detailed quote", "my packaging", and enter the size of the box you're using.
2) You got an online quote, but filled out a paper air bill. Online quotes include "online booking discount", but you get charged full price with a hand written shipping label.
3) As mentioned, your weight was off so they adjusted it by rounding up, or off because you didn't have everything in the box like cold/heat packs, packing peanuts, paper wads, bubble wrap, etc. when it was weighed. I used to always forget the cold pack when I weighed, doh!
4) The "residence delivery" box wasn't checked at time of quote, so more was charged as it wasn't a business address. I always ship to a FedEx staffed location now, so i never have to worry about this anymore.
5) Checking the box making it a "Must have signature" also raises the cost... also, not needed if shipped to a fedex location as "Hold for Pickup" as they make the receiver sign for the package anyway.
those are a few that I discovered since I was certified and started shipping... hopefully that helps!