之前也有类似的疑问，
跟母语人士讨论时有google到一篇别人的回文，觉得蛮实用的，
跟熟悉程度有关，参考看看
Group A: familiar
The (b) version sounds not quite right (or "unusual"):
1a. He's a friend of Jack's
1b. He's a friend of Jack
2a. He's a teacher of Jack's
2b. He's a teacher of Jack
Group B: quite familiar
Both versions sound normal; the focus in each is slightly different:
3a. He's a friend of my sister's
3b. He's a friend of my sister
4a. He's a friend of Mr Whiteside's
4b. He's a friend of Mr Whiteside
Group C: more remote
The (a) version sounds "chummy", perhaps oddly so:
5a. He's a pupil of Francis Bacon's
5b. He's a pupil of Francis Bacon
6a. He's a friend of the Pope's
6b. He's a friend of the Pope
Group D: very remote
The (a) version begins to sound inappropriate:
7a. He's a grandson of Queen Victoria's
7b. He's a grandson of Queen Victoria
Thus the movement on the scale is from a preference for the a) forms towards a
preference for the b) forms.
There is also another group, which does not relate to "familiarity":
Group E: ecliptic
Another meaning takes precedence; thus here in b) the meaning "someone who stu
died Kant" eclipses the meaning "one of Kant's students":
8a. He was a student of Kant's
8b. He was a student of Kant