The maths syllabus, unlike those for other subjects, isn't clear-cut between Yr 11 and Yr 12 topics. It's actually titled the "Mathematics 2U/3U Year 11-12 Syllabus". You can think of the topics/modules in other subjects as distinct blocks, but in maths you should think of them as layers (and with lots of overlaps, mind you).
Since you mentioned Cambridge Year 11 3U, you've probably noticed that the last 3-4 chapters (from memory: integration, exp/log funtions, trig functions) are not taught in Year 11 in most schools. I think Pender intended for them to be taught in Term 4 of Year 11, by which time we already call ourselves Year 12. So textbooks are only to be used as guides - they are by no means definitive representations of the syllabus. If you've looked at more than one textbook, you'll have seen that each author strucutures the topics in his/her book in a different order. Schools can also choose to teach them in different orders.
