Created attachment 338732 [details] demo using hlookup
"The search vector for the LOOKUP must be sorted ascending, otherwise the search will not return any usable results." Which is also the case for Excel (which presumably is where this misery originated), and Low Medium High Giant is not in alphabetical order. This can be pulled off with HLOOKUP and the flag for "don't sort" as in the attached example, but LOOKUP requires a sorted search vector.
*** Bug 583490 has been marked as a duplicate of this bug. ***