Looks like a cheaper version of a basically poor design.
From your diagram you adjust rod 18 at the pin 19 by turning the nut 20.
Because of the leverage effect, a small adjustment there will make a big difference to the overall height.
Similarly a small amount of wear will also make a big difference to the height
Same goes for rod 31 and the two plates it locates through.
Ditto for the holes in the deck hanger rails parts 36 & 41
These rails are held in by 4 bolts on each side through the foot boards which rarely come loose but they will also have a drastic effect on the deck height.