well Sir, I practiced from IDP book and same thing experienced by me. Their practice tests are somewhat difficult as compared to cambridge ielts book series. However, if we compare it with actual ielts exam, Let me tell you that My actual listening test which I cleared for my immigration purpose was bit more difficult than normally it supposed to be. I gave ielts three times before (2-3 years gap in between). but last listening test was more difficult than previous ones. This is one off event.
In the nutshell, yes I have to agree that reading and listening practice tests provided in IDP books are more difficult than normal. I even stopped practicing from it as I found myself demoralized after attempting a test from this book. But later on, I started preparing from it again as my confidence grew. So, if you have enough time available before your test, I will suggest you to prepare from cambridge ielts series books in the begging and later on shift back to IDP book, when you will become well versed with IELTS test format and tricks.
Regards
In my group all the friends who cleared IELTS from BC got clb 9 and All those gave through IDP did not clear CLB 9. They complained of noisy places selected by IDP and they did not have speakers working properly.
. In short practice with IDP material 
