Simplicity really. It would have been possible to offer those, or similar related units, but that's been left to more featured apps. So, with Bargain, if you have two products you are comparing and one is ₤2.38 for 800 grammes and the other is ₤3.50 for 1.2Kg, then enter 2.38 and 800 for one and 3.5 and 1200 for the other. The bargain will still be displayed.
This feature only works directly after a successful comparison. So, after you have tapped the Compare button and want to enter new values, shake your device. If the insertion point is in either Price or Quantity then the shake will be ignored.
Yes. You can read them here.