As explained in the Guild Operator's gLiveView section, served is:
served blocks counts how many blocks were fetched by "in" peers. If this does not increase for a long time, it means the "in" peers are learning about new blocks from somewhere else (and therefore this node is not contributing towards accelerating the propagation). Overall, these metrics are helpful in tweaking the topology and/or performance of the network links.
In other words, served is a measure of how many blocks have been fetched by other nodes from your node. It makes sense that served will be lower for your BP because the BP is supposed to only communicate with your relays, whereas your relays are responsible for communicating with the rest of the network.