I've had to do some exploratory & other surgery in the kitchen ceiling
(also bathroom floor) as a result of a loose connection in an old
junction box (kitchen & bathroom lights went off sometimes but came
back on if you stepped on the right area on the bathroom floor).
That's all fine now (Wago box; new ceiling light; big mess cleaned
up), but I wonder about the weird situation I found in the
ceiling/floor.

= bathroom floor
- kitchen ceiling
X joist


================================
XX XX
XX XX XX XX
XX XX XX XX
XX XX
--------------------------------

That's a cross-section. There seem to be two sets of joists: one set
has a gap above & the (lath & plaster) ceiling attached below; the
other has a gap below & the floorboards attached above. I've only
worked in the middle, so I don't know how the ends of the joists are
attached to the walls.

Any explanation for this weird situation?

Thanks,
Adam