How much extra carpet do installers need?
Carpet isn't paint.  You can't infinitely cut and splice carpet and put a bunch of seams in it, and expect to have a surface that will wear well.  

So you can't understand the problem until you know the dimensions of each space and also the dimensions of the supplied carpet. 

It's just like if you buy cloth to make a pair of pants, and your waist is 44 inches, so each panel of the pants need to be about 13" wide, except the cloth you like comes in 40 inch wide bolts. You will end up wasting almost half the cloth. That's just how cutting and fitting works. 

The installers are probably planning to run the carpet linearly down the hall so there are no seams in the hall, then do the side rooms with remnants if they are not able to fit it direct.