I don't follow Bitcoin's testnet closely, but I assume it's because the difficulty drops back to 1 whenever no block has been found for 20 minutes [1]? At least that would explain testnet's block height of beyond 1.6M [2] which means it already went through 7 halvings which would correspond with the block subsidy mocacinno mentioned.

[1] https://en.bitcoin.it/wiki/Testnet
[2] https://live.blockcypher.com/btc-testnet/