It's confusing because we're talking 2 different things:
The zoom range is 3X (210/70).
The magnification factor, at 210mm, is 4.2X (210/50mm, a standard lens).
However, a digital camera usually has a focal length maginifaction factor depending on the sensor size, probably 1.5 or 1.6, so a 70-210 zoom is really like a 105/315mm, so the zoom range will still be 3X, but the magnification factor will jump up to about 6X.