Assalam Alekum Respected Brother.
I'll try to find the same hadith with simple words that will help people understand in a better way Inshallah.I just did'nt even think that this can be difficult for anyone to understand because every one knows that fasting in Ramadhan is Fardh while fasting in other months is Nafl.So there is no need to mention Muhammad SAW fast the whole month of Ramadhan but they are talking about other 11 months.
This would be more simple,i think.
A group of scholars, including Ibn al-Mubaarak and others, thought that the Prophet (peace and blessings of Allaah be upon him) did not fast all of Sha’baan, but he fasted most of it. This is supported by a report in Saheeh Muslim (no. 1954) narrated from ‘Aa’ishah (may Allaah be pleased with her), who said: “I never knew of him – meaning the Prophet (peace and blessings of Allaah be upon him) – fasting for any entire month apart from Ramadaan.” According to another report also narrated by Muslim (no. 1955), ‘Aa’ishah said: “I never saw him fast for any entire month from the time he came to Madeenah, apart from Ramadaan.”
Jazakallahu Khyran for pointing out.