From what I understand, it has to do with 1) the time displayed is for an "optimal" load (not too full, normal soilage, balanced, etc.) instead of the actual load, and 2) the machines may pause and restart or lengthen parts of the cycle if the weight and/or balance are slightly off.
In any case, the washers at my apartment complex are always off, but it's at least consistent so that I can add about 6 minutes to whatever it shows at the beginning and plan around that.