I googled it and came up with this link;

http://www.ssina.com/faq/index.html

The magnet test is find out if there is a layer of magnetic steel in the bottom layers that make up the pot. This indicates that the pot is likely suitable for use on an induction cooktop. The test has no other use because as you have discovered, some types of steel are magnetic and others aren't.

Just a few rules to keep in mind. The numbers on stainless steel products refer to the percent of materials used to produce a specific alloy. Using 18/10 as an example. The 18 refers to the % of chromium in the finished steel. 10 refers to the % of nickel. 18/10 is defined as Austenitic or, more commonly, 304 grade stainless. In consumer applications it provides the strongest and corrosive resistant material.

The reason the magnet stuck to the base is becasue the manufacturer selected an alloy with no nickel. This steel is called 18/0 stainless (430 grade). The reason this is commonly used is that it still contains 18% chromium which provides strength and resists corrosion. The absence of nickel however makes the cookware suitable for induction (magnetic) cooktops. Not surprisingly, it's also less expensive than 18/10 stainless.

While 18/0 stainless steel is acceptable for impact bonded cookware bases and multi clad exteriors, It should never be used as the actual cookware interior surface. While the chromium is corrosion resistant, the lack of nickel makes the steel very suceptible to pitting. Only use cookware with an interior surface of 18/10. If a magnet does not stick to the interior pan wall, you're fine.