I thought the same as you regarding too many custom style entries, but it happened again yesterday after I had cleaned them out.
BTW:
Here is the VBA macro script that you can run to clean up the custome style list.
Sorry could not indent properly here, but create this in a macro. A little slow, but works.
Found this in another forum.
Sub TrimStyles()
Dim s As Variant
Dim g As Style
On Error Resume Next
With ActiveWorkbook
For Each g In .Styles
If g.BuiltIn = False Then
s = g.Value
.Styles(s).Delete
End If
Next g
On Error GoTo 0
End With
End Sub
Greg