I think the fact the IB is considered superior(I am studying IB too!) is not only the number and depth of the subject they need to take, it is also due to the core section of the IB course, ie. TOK, CAS and EE. I think the IB course shows the student as a all-rounded person instead of a studying machine. They need to fulfill all those requirements in CAS, write a formal (but short) essay in (limited) depth, and also think philosophically in TOK.
Oxbridge(or specifically cambridge here) loves students who wrote formal essays. I have heard that (from my teacher who studied in Cambridge) that students from Harvard who went to Cambridge as exchange students needed special courses to learn how to write essay, but writing research essays like the EEs is essential in the education of Cambridge.
TOK is also one big factor that makes IB superior. Behind all subjects there are philosophies behind. Cambridge education wants people who not only learn the facts and print it out, but also think critically. TOK trains critical thinking(supposedly...).
They also wanted all-rounded students who studies everything. IB forces students to study at least 1 humanities, 1 science, 2 languages and 1 mathematics subject. So students will not only focus in one single direction(perhaps in science-only subject as people do in A levels), but learn to see the world in more aspects.
So its not only the subject choices and depth that counts.