What does the following Hadith mean?:

Narrated by ‘A’ishah (rta): Allah’s Messenger (sws) said: “Whoever died and he ought to have fasted, then his guardians must fast on his behalf.” (Bukhari , Kitab al-Sawm)

More specifically: Why should the guardians be responsible?

If all the texts of this Hadith and other similar Ahadith are collected and analyzed, it comes to light that here the fasts of Ramadan are not implied. The fasts mentioned here relate to vows. In other words, the Hadith means that if a person had pledged to keep some fasts and was unable to keep them because he died before he could, then one of his heirs should do so. This is much like a debt outstanding in a person’s name, which, we all know, is passed on to the heirs if the actual person dies. Consequently, the Prophet (sws) is reported to have said to the daughter of a deceased who had died and had left avowed fasts:

“Had she had an outstanding debt in her name, would you not have paid it.” She said “Yes”. At this the Prophet replied: “It is more befitting to fulfill what is outstanding to Allah.” (Muslim, Kitab al-Siyam)