A fairly good explanation for the differences can be seen at https://en.wikipedia.org/wiki/Coastline_paradox
The length of a coastline depends on how you measure it. It can be how far it takes to sail round it, how far to walk round it with one of those little wheelie things, or how far by running a tiny wheelie things round the outline on a map. Or if you are doing it by higher methods, it involves fractals. Does a bay like, for example, the narrow ended Mersey Estuary count as coastline, and if so, how far up do you count it? Or take the Bristol Channel and Severn Estuary. Where do you draw the line between sea and river? And what about islands you can walk to at low tide? Where you have cliffs, tidal rise and fall is a vertical consideration. Where I live, tidal rise and fall is more a matter of horizontal mileage, although the vertical element does come in when a vehicle is stuck in the sand over a mile out (the headlamps were a pinprick of light in the distance, and went out as we looked).