My room is 400 square feet, but we’re being told to order 450 square feet of flooring. Is this really necessary?

Yes. As a general rule, industry standard says you should plan to order about 10% more flooring than is needed for the installation.  

You may need to order slightly more or less depending on the room. For example, if you need to work around stairs, a bay window, a fireplace, and a closet, you may need to have more than 10%, but if the room is square with no interruptions, less than 10% may work. 

 There are many factors that can contribute to the amount of material needed for a job.

Things like carpet and vinyl that come in a roll, only come in certain widths and the material is directional, meaning it has to be laid in one direction. If you have ever noticed that carpet changes color depending on which way you vacuum it, then you have seen what we mean. If you have a room that is 17.6' wide and choose a carpet that is only 12' wide, you will have waste where we have to cut the carpet to fit. If you choose a carpet with a pattern, we have to match the pattern at the seams and that, too, can cause more waste. Your Lowe's specialist will have the diagram of your room we provided to them after we measured. You can always ask to view it so you can see in advance exactly what we have to do. 

Hard surface items, like wood and tile, have an industry standard waste factor added to the job. This accounts for any bad pieces that may be hiding in the box, difficult cuts where only a small portion of a board can be used, etc. Much of the material will be cut to fit the exact space, and once the boards are cut, they likely cannot be used elsewhere in the room because the end tongue or groove will have been removed. Once that happens, that board can no longer adjoin with another board, so there is some waste involved.

Remember that the more intricate a job you have, be it from the shape of the area to be installed to the type of material you choose, the more waste you will have.