What's the deal with paying 9/10ths?

We just bought a tank full of gasoline (petrol for those of you in Ireland) and it costs us 38 cents.
 
Not per gallon, that would be a rip off.  Oh no, we paid 1.9 cents per gallon X 20 gallons...which we all know comes out to $.38
 
Not long ago I posted a receipt that was roughly $12 for about $122 worth of groceries.  Turns out that store has bonuses that are paid in the form of gasoline discounts.  We had amassed $2.55 & 9/10ths per gallon in discounts, and gasoline was $2.53 yesterday. 
  
Unfortunately they only gave us credit for $2.54!!!
  
And that's why we had to shell out a quarter, dime and three pennies to fill up the miniature van.  It's ok.  I'm not going to call and complain about the screw up with our card.  I'm willing to overlook the .0019 screwup.  I'm even willing to eat that cost to avoid embarassing the grocery mart. 
 
It's time we tighten up our belts and just move on. 
 
God bless America
& Oprah...that just goes without saying...but I had to at least write it...