From what I am understanding, you should consider three coordinates, which are going to be different, with different numbers, but they will place each one in its context at the same point on the earth:
Geographical coordinates, latitude and longitude expressed in decimals of degree.
You could use them as an equirectangular projection, which we know as flat lonlat (EPSG: 4326). But even if you do not use it as a projection,
LatLng is the geographic coordinates of a point.
Here it seems that you should have entered a feature in geojson.io and you should have entered it in geographic coordinates. But the map of geojson.io is projected in Spherical/Web/Pseudo Mercator (EPSG:3857), beyond that it is necessary to enter the geographical coordinates. You realize when you see a Mercator projection by the shape and size of Greenland. In fact, I do not know a web map that is not projected in that system.
Here you get the geographic coordinates of the northest eastern corner of your map bounding box:
Coordinate reference system, EPSG:3857 by default, that is Web Mercator.
Web Mercator has two properties that make it useful for web applications: It is based on a sphere, therefore the calculations are much simpler than if they were made on an ellipsoid. And it is still a Mercator projection, in the sense that the loxodromes are still seen as straight lines.
Its units of measure are meters on the Earth's surface, therefore you should expect high numbers for the coordinates. The x coordinates are simply calculated as
lambda * R, where lambda is the geographic longitude expressed as radians, and R = 6378137m, the radius of the sphere. In this way the meridians are seen as vertical lines. For the projection to be conformal, a deformation is applied in the latitudes, and that is the reason for that dimensions for Greenland. In addition, this deformation tends to infinity at the poles, so the projection is cut close to the latitudes -85 and +85, forming a square of 40075016m width by 40075016m height (coordinates are from -20037508 to +20037508, in both easting and northing).
In both of your previous questions you had an approach to the projected coordinates of your map.
Represents a point with x and y coordinates in pixels.
It is assumed that depend on the CRS and the zoom level. Here it seems that you already understood its operation.
It is clear that these numbers will be smaller than the coordinates of the projection as the zoom approaches the map.
But the main difference between the three systems is that they are measuring different things on the same map: decimals of degree of geographic coordinates, meters projected from a sphere to a cylinder with a particular analytical deformation, and pixels of an image.
Sorry I do not know Leaflet and can not help you more about it. The above is deduced from what I was reading about your previous questions and the documentation. I hope it serves to clarify the concepts a bit.