The network is either working or not at any given
instant. During that instant, CAP determines how the system responds.
Reliability is the graph of many instants over some period of time, so 50% reliability over a day means it's not working roughly half of the instants measured for 24 hours.
So it doesn't whether your network is 1%, 99%, or 100% reliable because CAP is saying that when a partition exists, this is how the system works. If the network is 100% reliable, then sure, partitions never happen and you can be CA, but since that is not possible and partitions will always happen eventually, you have to be either CP or AP - and Spanner chooses CP, which is the only accurate description.