Are you running LND or Core Lightning? Can you compare the results with what you get from the direct Lightning API? (by running the commands below in the BTCPay Server directory)

For LND we get those stats via the channelbalance command:

./bitcoin-lncli.sh channelbalance

For Core Lightning it's listfunds:

./bitcoin-lightning-cli.sh listfunds

I assume RTL and BTCPay Server account differently for the separate types of funds that might be in HTLCs that are processing.