Why do I always run out of paint?

The bathroom is nearly finished and I'm out of paint.  How can such a little bitty room take so much paint?  I think I only need another pint.  Of course if I only buy a pint it will need a pint and a half.  The regular rules of math must not apply to paint.  It never covers the calculated area of the wall...minus the windows and doors...times 2.5 coats... Wait!  I always forget to calculate the quantity of liquid the average paint roller retains no matter how hard you roll.