If you set the tx fees as 100 bits / kilobyte, even if the transaction is substantially less than a kilobyte it will always pay a minimum of 100 bits of fees. This translates to often be x3 what you would expect it to be, and not consistent with the priority code.
This means:
bitcoin-cli settxfee `bitcoin-cli estimatefee 2`
is very unlike actually having the estimated fee target as 2.
Another side effect of this, is that the when using paytxfee it has an extremely unpredictable fee priority, as the true fee/kilobyte can easily vary by a factor of x3. When using paytxfee, I've noticed some transactions confirm in the next block (for fee reasons) while others have been stuck >5 blocks