In AIP letter, it says:" Please note that a final decision will not be made until all remaining requirements for becoming a permanent resident have been met. These requirements include medical, security and background checks for you and, if applicable, all of your family members, both in Canada and abroad, even if they are not applying to join you in Canada at this time."
I don't know about the interview but, guess is a background check and eligibility check are two different processes? If your background check is complicated: been to many countries, dependents? other histories?
Either way, if there is nothing, then you have nothing to worry about, it is just regular delay. I totally feel your worry, but it will all turn out just fine!