[MarkLogic Dev General] stemmed searches

[Whitby, Rob, CMG]

Can anyone explain why these 2 queries return different results?

count(
  cts:search(
    /Journal/Volume/Issue/Article/PublishDate/Year, 
    cts:word-query("2009", ("unstemmed"), 1)
  )
)

= 3036 (the correct result)

count(
  cts:search(
    /Journal/Volume/Issue/Article/PublishDate/Year, 
    cts:word-query("2009", ("stemmed"), 1)
  )
)

= 2757 

Why is the "stemmed" setting causing some matches to be missed?

Thanks
Rob