lightning network – The total number of channels differs from that of explorers

I have my own c-lightning knot. listchannels returns 11,455 but explorers, for example https://explore.casa/, display about 39,000. Do I have to define anything to get all the information on the channel?

$ lightning-cli listchannels | source of grep | bathroom
11455 22910 996585

I think that "initial_routing_sync" could be relevant. But I do not find how to use it.

When receiving an initialization message with the initial_routing_sync flag set to 1:
SHOULD send gossip messages to all known channels and nodes, as if they had just been received.

https://github.com/lightningnetwork/lightning-rfc/blob/master/07-routing-gossip.md