There is no certainty with this sort of thing. It depends partly on dosage and duration of use but mostly on factors related to clearing it from the body. This would be the activity of certain enzymes in the liver that are involved in conjugation and removal of the steroid from the body. Of course there will be individual differences in metabolism based on genetic and environmental factors and thus I think that the range given is appropriate.
Remember that these guidelines are for when you are likely to test negative, not that there is absolutely no exogenous steroid in your system.