That depends on the royalty rate yu select and where you sell your book!
If you selected the 70% rate it is only good in the "parent" country I.E. the 70% rate for the .COM extention is only good for the USA if the sale goes to Mexico or Australlia you only get 35% which is probably what happened.
The USA based Amazon.COM services about 75 % of the world
the other domains -- the .UK, .FR, .IT, .ES, .ETC.... Only service their countries and not much else thes also provide the 70% rate but the UK is probably the only area in which you will sell.