As you can now see the fuel is picked up (and returned) from the well in the bottom of the tank which is covered by a metal plate to stop the fuel from surging out of the well when cornering/braking/going up and down hills.
The well is filled only via the hole in the top.
When the tank is near empty the four baffles direct the last remain fuel swishing about over the hole.
The fuel sender float sits in the front left corner of the tank hence why it is prone to variations when the tank is not level, low fuel reading when going up hill etc.

Inside the top of the tank.
