Nasai

1790. Commentary: The Messenger of Allah (sallallahu alayhi wa sallam) would generally perform eleven rak‘ahs, but whenever, due to the aforementioned reasons, he was unable to perform this night prayer, he would, during the day, add one rak‘ah to make them even, and thus perform twelve rak‘ahs. If he were to perform ten instead of eleven, there would be a deficiency in the supererogatory prayers (nawafil), but he (sallallahu alayhi wa sallam) did not prefer any deficiency.

English Commentary:
1:
In Sahih Muslim, it is narrated from Ibn Umar radi Allahu anhuma that he said:
Whoever’s night voluntary prayer (tahajjud) is missed,
and he performs it between the rising of dawn and before noon, he will not be written among the heedless.
Perhaps twelve (rak‘ahs) were sometimes prayed for this reason: that the witr had already been performed after ‘Isha that night,
and witr is not performed twice.
Or, on those nights when it was anticipated that the full eleven rak‘ahs could not be performed, witr would be performed during the night itself.
Or it may be that when the Prophet sallallahu alayhi wa sallam would pray twelve rak‘ahs during the day, he would make the witr even (i.e., add one to make it an even number).
And Allah knows best.