What does Sphere really mean?

Just a quick one to deal with an issue in a recent customer order.  The customer requested reading glasses but entered a Sphere correction that started with a "-".

The “Sphere” column indicates your level of long or shortsightedness, a “+” indicating Hypermetropia (long-sightedness) and a “-“ indicating Myopia (short-sightedness).

For reading glasses we'd expect to see a "+" measurement for the Sphere column or a measurement in the ADD column.