It seems that LOOKUP is finding a close match. Instead of LOOKUP, use VLOOKUP (to search columns) or HLOOKUP (to search rows). Those functions have "Close match" or "Exact match" parameters.
The Function button on the Toolbar will allow you to choose Function Browser to see the syntax of functions.
I am using Numbers'09 version 2.3 on a Mac running OS X 10.8.5
I have no idea how Numbers 3.x works .