The Structure stat shows the Reactor HP (of the current active reactor and its chambers) when you're using the new system, and it shows the old Structure HP if you're using the old system.
2 issues here:
# A few blocks: Power, Stabilizer and Conduits do not have any Reactor HP
# All of the old Structure HP values were set to 0, something that should not be done if you want to allow people to switch from new to old reactors.