Total channel count differs from explorers
I thought we had that question before but the only one similar I could find was this one 1ML shows larger number of nodes comparing to other lightning network explorers which was not quite the same.

So to answer your question: listchannels depicts information your node knows from the gossip protocol. If your node is new this could take some time which is most likely the reason for your lower number. 

I once had a corrupted gossip_store file in .lightning folder after upgrading a node in that case shutdown your node delete the gossip store file and restart for a new sync. Pay attention not to delete any other files in that directory as they are critical to access your funds