The TxnCounter was not part of the consensus protocol when we launched, which is why you don’t see it until round 3317192 on mainnet. In addition, there were some empty blocks before it. The value would have been omitted until the first transaction after the protocol upgrade.
It looks like there was one transaction on round 3317192, which is why you see the value is 1. There were no transactions on round 3317193, so it remained 1. There was one transaction on round 3317194, so it was incremented to 2. Etc.