That is an impact of the channel reserve, see: https://docs.rs/lightning/newest/lightning/util/config/struct.ChannelHandshakeConfig.html#structfield.their_channel_reserve_proportional_millionths.
The channel reserve is enforced to make sure every counterparty at all times has one thing to lose, in the event that they have been to attempt to cheat. By default, the channel reserve is 1% of the channel capability, and LDK units a tough minimal of 1000sats.
Due to this fact, should you have been to ship sufficient such that greater than max(1% of channel capability, 1000sats) have been in your counterparty’s facet, your obtainable steadiness for obtain would improve.
