So actually the interesting thing is that Iquidus (the base for the block explorers) can calculate the total coin supply in several different ways. Here's what the config says:
// how to calculate current coin supply
// COINBASE : total sent from coinbase (PoW)
// GETINFO : retreive from getinfo api call (PoS)
// HEAVY: retreive from heavys getsupply api call
// BALANCES : total of all address balances
// TXOUTSET : retreive from gettxoutsetinfo api call
I assume that the gettxoutsetinfo call is the most accurate one, but to match the other explorers as closely as possible, I didn't end up using that in mine. Togo and I noticed some differences between the 3 explorers and the api call, but the closest was COINBASE.
If you look at
https://explorebiblepay.com the total supply is reported via the COINBASE (PoW) mechanism. I'd be happy to change it to txoutset, which I still think is the most accurate, but then we'd have 3 explorers with 3 wildly different reports of supply. I haven't tested this myself, but I am willing to guess that biblepay-central is using GETINFO (PoS) instead.
Perhaps Togo/Licht/myself should get together and agree on a way of reporting supply within the block explorers.