Why does this query a key lookup?

Question

I am trying to understand why does this query a key lookup, even if I added all columns i am selecting in the index?

Here's the query

SELECT TOP 1
    ART.Id, 
    ART.Title, 
    ART.InitialTitle, 
    ART.Summary,
    ART.IsCategoryFirst,
    ART.RootCategoryId, 
    CAT.Name, 
    ART.OldFacebookCommentsUrl, 
    ICO.CssClass,
    ART.TopicName, 
    ART.MainArticlePhoto, 
    ART.FrontPagePhoto, 
    ART.PublishDate
FROM Articles ART WITH (NOLOCK)
INNER JOIN Categories CAT WITH (NOLOCK) ON CAT.Id = ART.RootCategoryId 
LEFT JOIN ArticleIcons ICO WITH (NOLOCK) ON ICO.Id = ART.IconId
WHERE ART.RootCategoryId = 3
  AND ART.PublishDate < GETDATE() 
  AND ART.Active = 1
  AND IsCategoryFirst = 1

Here is the index which should cover it

CREATE NONCLUSTERED INDEX [SmallArticleResponse] ON [dbo].[Articles]
(
    [RootCategoryId] ASC,
    [Active] ASC,
    [PublishDate] DESC,
    [Coefficient] DESC
)
INCLUDE (   [Id],
    [Title],
    [InitialTitle],
    [OldFacebookCommentsUrl],
    [Summary],
    [IconId],
    [TopicName],
    [FrontPagePhoto],
    [MainArticlePhoto],
    [FacebookPhoto],
    [IsCategoryFirst]) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

This is execution plan and info for it.

Answer 1

Because IsCategoryFirst column isn't in the key, most likely. And/or because there is a separate (single column?) index on IsCategoryFirst and it's using that instead.