Relative humidity is the water vapour in the air as a proportion of the maximum that air could hold at its current temperature. The phrase "at its current temperature" is the part that matters in a humidor: warm the box and the same amount of water becomes a lower reading, so temperature and humidity cannot be set independently of each other.
What does 70/70 actually mean?
70 °F and 70% relative humidity — a shorthand inherited from an era of less precise instruments and less climate-controlled shipping. Many keepers now run lower, commonly 62–65%, and the reasons are practical rather than doctrinal.
Why lower is usually safer
Mould risk rises sharply above roughly 72%, and an over-humidified cigar draws tight and burns unevenly. The opposite failure is real but slower: below roughly 58% the wrapper becomes brittle and can crack, and volatile oils dissipate. Between those two, the range is wide and the exact number is a preference.
What holds it steady
Some source of moisture, some means of measuring it, and a box that is not itself pulling water out of the cigars. Two-way humidity packs hold a fixed figure without adjustment; a refillable device needs distilled water and attention. A hygrometer that has never been calibrated is the most common reason a humidor is not where its owner believes it is.