The incredibly confused mathematical diagrams.
Sorry, will try explain to you, makes it easy:
Blue = earth (with round earth assumed radius of 6371km)
Red = causeway (with supposed length of 38.422km this says wikipedia)
Blue dot and 116m Height difference is how much lower the end of the causeway is, if you look straight ahead from the start, along flath earth line.
If earth were flat, which I see, the height difference is zero (0m) of course. But round earth assumes it drops away, and simple calculation below show it will drop away 116m.
Explained makes better understanding, no?
Or do you think earth is not circle? (round earth assumption)
Or earth is not 6371km radius? (round earth number)
Or do you think causeway is different length? (maybe wiki is wrong)
Or do you not think it will drop away? (you believe in flat earth, great)
(new picture, zommed in even further)