Odd Full Text Search Result

Hi I have 2 following 2 queries:

SELECT FT_TBL.JobId, FT_TBL.Title, FT_TBL.[Descri开发者_如何学编程ption], 
     FT_TBL.Location, KEY_TBL.RANK FROM Jobs AS FT_TBL 
INNER JOIN FREETEXTTABLE (Jobs, (Title, [Description], Location), 
    'asp.net software')
    AS KEY_TBL on FT_TBL.JobId = KEY_TBL.[KEY]
WHERE CONTAINS (Location, '"luton*"')
order by KEY_TBL.RANK desc;

and

SELECT FT_TBL.JobId, FT_TBL.Title, FT_TBL.[Description], 
    FT_TBL.Location, KEY_TBL.RANK FROM Jobs AS FT_TBL 
INNER JOIN FREETEXTTABLE (Jobs, (Title, [Description], Location), 
    'asp.net software', 100)
    AS KEY_TBL on FT_TBL.JobId = KEY_TBL.[KEY]
WHERE CONTAINS (Location, '"luton*"')
order by KEY_TBL.RANK desc;

The only difference is the second one has a top_n_by_rank parameter set to 100, the first one returns 1000+ results, I thought the second one would return 100, but it returns 0 results, why is this? How should I change the top_n_by_rank parameter to make sure I get only the best results?

The top 100 results probably don't match "luton*, and get filtered away by your WHERE clause.