settxfee: formula for fixed fee
Big disclaimer: you really shouldn't do this. Transaction prioritization works based on fee per size, so that's what you should be configuring.
As for an explanation of what you're seeing: the block explorer you're using is showing the wrong size. For fee computations what you need is the vsize, not the serialized size. This is the size that takes the discount for witness data introduced by BIP141 (segwit) into account.
The vsize of your transaction is 141, not 222.