Because cyberware scanners have to beat a certain threshold in order to detect the item. MAD scanners just need one. Per page 262 of SR4A, standard cyber or a weapon just needs 1 hit on the cyberware scanner. Higher grades have a higher threshold, all the way up to "5+" for Delta gear.
So, basically, a rating 3 MAD scanner will, on average, pick up whatever is being hidden. A cyberware scanner will have a harder time with the higher grades, even with a larger device rating. The cyberware scanner is more versatile, however, since it can pick up more details. However, the MAD scanner is not vulnerable to jamming like the cyberware scanner: the CS uses radar and advanced software, the MAD scanner uses magnetic fields like a blunt instrument compared to the CS.