This is easily done, as any perceived benefit from the additional lenses given is often on the fringes of making any real difference in your eyesight. It might be very minor, but once the optometrist is aware there is some improvement in eyesight, no matter how small, often the additional diopters remain. It is not uncommon for optometrists to correct to more than 20/20, such as 20/15. This may be done to give you the best possible vision at all times, including low light conditions such as night driving. The problem with this is that the majority of myopes spend a lot of time wearing these powerful glasses up close, which contributes to hyperopic defocus.
Standard lens are only available in 0.25D increments, so the optometrist cannot give you the exact strength you need. Rounding towards the weaker strength would introduce (a very slight) blur, so it's not unreasonable (from their perspective) that they might choose to round towards the stronger end.
Overprescription is very easy to correct by wearing less diopters. The wearer never needed those diopters in the first place.