This is because in holiness there are only ten elements; ten not nine; ten not eleven. Whereas by contrast within the Sitra Achra there are eleven crowns of impurities. This is also why regarding the Ketores there were eleven ingredients. This is in order to refine and elevate the eleven crowns of impurities; and the reason why the Sitra Achra has eleven elements is because they also include the vitality that sustains them as a standalone entity.
This is because, in holiness the vitalising energy of the ten supernal Sefiros is drawn internally within each of them and unites within them and therefore is not counted independently. This is also true for every other aspect of holiness, because, every aspect of holiness is comprised of the number ten. Whereas, in the Sitra Achra the life source that vitalises them is independent of them [for if it would allow itself to be drawn within them, it would be swallowed up within them and would not be able to vitalise them and therefore it maintains its independence and is counted separately.]
And thus the Sitra Achra is the antithesis of 'Kedusha' and whatever exists in the antithesis must also exist in Kedusha. From this it is understood that since in the antithesis there is an eleventh element which is separate from the other ten, this must also exist in 'Kedusha' too opposite it where there is a level which is higher than the ten supernal Sefiros and is separated from them. {Except, in 'Kedusha' the eleventh entity is not the life source of the ten supernal Sefiros (because the life source of the ten supernal Sefiros pervades within them and unites with them, as mentioned earlier.)}
However, the state of 'Makif' which is positioned above the ten supernal Sefiros is a standalone entity and therefore is not counted together with them as in אַּנְתְ הוּא חָד וְלָא בְחֻשְבָן ‘You are one but not part of the number’.
However, the Makif aspect of the Sitra Achra is their life source which is why the Makif needs to be included in their count (for without the 'Makif' they have no life source) and that is why they are eleven in number.} Accordingly, it is understood, that the refinement of the eleven crowns of impurities is through drawing down the 'Makif' which is positioned above the ten supernal Sefiros which does not get counted together with them (the state of 'Atik') analogous to the eleventh state of the Sitra Achra which is separated from them, as mentioned earlier.
