abednego It's the latest GKI LTS revision with changes from after 6.1.155 but not 6.1.156 or later yet. GKI LTS update often don't include version bumps. It's partially 6.1.156 but not all the way there. This is the latest GKI LTS branch maintained by Greg KH, that's just how it is right now.
We're shipping the standard GKI LTS revisions from Greg KH rather than doing the merges of stable LTS kernel releases ourselves as we used to do before. It's not really supposed to lag behind but Google doesn't really care and hasn't put more resources into it. Too much of it is left up to Greg to do on his own and that's why it lags behind. They keep focusing on cutting costs and laying people off so don't expect it to change any time soon. We can do it ourselves again when we have more resources but it's a waste of our time to be doing the same work Greg KH is going to do to be a week or so ahead of where it is.
This happens because of the awful model put in place for GKI of having a stable ABI which has absolutely no benefits for the stock Pixel OS or for us, only downsides by slowing down the kernel updates. It was done this way to make it easy to update the base kernel without updating drivers, but the Pixel stock OS builds the drivers with the current kernel that's being used and so do we so it's pointless. Other OEMs don't really take advantage of it so all it really ends up doing it burning time and slowing down updates. It's one of many cases of Google trying to cater to OEMs not keeping up with updates in a way that screws things up as a whole.