Ash-Shawkaani (may Allah have mercy on him) said:
“We have quoted above hadeeths which highlight the virtue of doing good deeds in the first ten days of Dhul-Hijjah in general, and fasting is included in that.
With regard to the report narrated by Muslim from ‘Aa’ishah, according to which she said: I never saw the Messenger of Allah (blessings and peace of Allah be upon him) fasting during the ten days, the scholars said: What is meant is that he did not fast those days for a reason such as sickness, travel and the like. Or the fact that she did not see him fasting does not necessarily imply that he did not fast. Moreover, it is proven that he said words indicating that it is prescribed to fast on those days, so the suggestion that he did not do it does not undermine the hadeeth.” (Nayl al-Awtaar, 4/283)
It may be that the Prophet (blessings and peace of Allah be upon him) did not fast these days on some occasions, because he liked to do things sometimes and refrain from them sometimes for fear that they may be made obligatory for the ummah, as in the case when he stopped praying Taraweeh in congregation in Ramadan. By refraining from doing something and taking a lenient view concerning it he showed compassion towards the ummah.
May Allah accept it from you and us.