Hi Bob ,
Probably it is not intuitive when we deal with money , since all of us would like to end up with more than what we started off with !
Apply the same principle to distances and see where it leads :
Suppose we have to travel 100 kms ; we travel 20 kms in one hour , and we are left to travel 80 kms , which is the first balance.
In the next one hour we travel 30 kms , which means our balance is now 50 kms.
If we now add 80 and 50 , we will probably give up travelling , since after travelling for 2 hours , and moving 50 kms towards our destination , we are farther from our destination than when we started !
Narayan