I think it has been discussed before but a lot of different issues are taken into account when upgrading customers. For instance they may look at your previous upgrade history etc. as well as your CIV.

They don't want a situation where people on certain routes, where for instance, certain cabins are regularily overbooked and then some people with a high CIV, book a lower cabin, knowing that they will always be chosen first for an upgrade.

So it is not always the person with the highest CIV that is upgraded.