Calculating Exact Map Size

I’m adding to this, OLD thread but its logical to place it here and sorry Extile Ihave no idea without more research how PS units compares ;l)

My question:

8129 Meters = 8.129 Kilometers.

^^ that as above, does not ‘match’ the ‘KM’ I see in left view of my terrain in viewport.

is that a known issue of the ‘viewport’ measurement, or am I missing something here ? ;))