I believe the left total colum just counts active parts so non-disrupted/-damaged actively taking part in battle. Losses, especially on retreat, may also account for damaged left behind (repair yards overrun)
WitE dev team - (aircraft data)
WitE 1.08+ dev team (data/scenario maintainer)
WitW dev team (aircraft data, partial data/scenario maintainer)
WitE2 dev team (aircraft data)
Damaged element when destroyed is counted twice 1) as normal kill and 2) destroyed damaged. Since elements can change state from ready to damaged during the course of the battle, total destroyed can go higher than initial element count when "destroyed damaged" check box is on.