In order to explain this, you'll need to understand that on a technical level, a bitcoin does not exist as a 'balance at some address', instead the system uses 'unspent transaction outputs' (UTXOs). See this question for more info.
The reason it is especially relevant to understand this, is that due to an apparent bug in the bitcoin codebase, there is no UTXO for the genesis block reward, which means the genesis block reward is un-spendable.
So with that in mind, we can look at each of the examples you listed.
66.92700504 BTC
https://bitref.com/1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa >https://www.blockchain.com/btc/address/1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa
These block explorers are listing the balance of the address that the genesis block coinbase transaction pays out to, and including the 50 BTC block reward in their report.
16.91700504 BTC
https://insight.bitpay.com/address/1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa >https://blockstream.info/address/1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa
These block explorers are apparently only listing UTXOs that exist and are spendable. The block reward is not spendable, so it has not been included in the sum.
66.92700504 BTC - 50 BTC = 16.91700504 BTC
and many others who don't sum two transactions:
for 50 BTC >https://www.blockchain.com/btc/tx/4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b?show_adv=true
and 0.1 BTC >https://www.blockchain.com/btc/tx/3387418aaddb4927209c5032f515aa442a6587d6e54677f08a03b8fa7789e688
These two URLs link to specific transactions, not to the address itself. The first is the coinbase transaction (though as mentioned above, there is no valid UTXO for this), and the second is a transaction creating a 0.1 BTC UTXO that is spendable by the owner of the 1A1zP1eP... private key.
