Words stylenames can be hidden from view quite easily so it is possible to have a style in the document which doesn't appear in the list because it is hidden. Normally, you go into Manage Styles to set the visibility but Table Styles can have this setting disabled for some reason that I don't understand.
Fortunately, vba comes to the rescue. This macro hides EVERY built-in table style (apart from one) and shows all the custom ones. I recommend you hide all of the built-in styles and then selectively show the few that you do want.
Code:
Sub HideMostTableStyles()
Dim sty As Style
For Each sty In ActiveDocument.Styles
If sty.Type = wdStyleTypeTable Then
Debug.Print sty.NameLocal, sty.BuiltIn
sty.Visibility = sty.BuiltIn 'only the custom names are visible
End If
Next sty
ActiveDocument.Styles("Table Grid").Visibility = False 'in magical Microsoft-land Visibility=False means you can see it
End Sub