Found this on GitHub:
It's not a mistake, it's literally a thousandth of a satoshi. A satoshi is the smallest unit for bitcoin, but lightning can transact with even smaller units while channels are open. The amount is rounded down to the nearest satoshi when the channel is closed and broadcast to the blockchain to adhere to bitcoins limit.

MilliSatoshi are the native unit of the Lightning Network. A milli-satoshi is simply 1/1000th of a satoshi. There are 1000 milli-satoshis in a single satoshi. Within the network, all HTLC payments are denominated in milli-satoshis. As milli-satoshis aren't deliverable on the native blockchain, before settling to broadcasting, the values are rounded down to the nearest satoshi.

All of the above however doesn’t give a definitive answer to the question, why?