As TT737FO more or less explained, RWY's are referred to by their MAGNETIC heading. If the heading of a RWY is 186°, it'll be called RWY 19.
Due to magnetic variation the magnetic heading of a RWY will change over time. This might result in a RWY heading of for instance 185°, thus causing that runway to be renamed RWY 18.
Did you know that in aereas of big magnetic abnormalities (e.g. close to the poles) we do not refer to a MAGNETIC heading but to a TRUE heading when giving a number to a RWY?
Well, it's actually clever isn't it? Wouldn't it be confusing to rename your RWY every week?
Im not 100% sure about this, but I think these RWYs also have a letter T in their name to make sure the pilot does not get confused. (e.g. RWY 19T)