Rat 5,
The rest of what you can see is made up of what you can't see
When you look out of the window you're looking down at an angle: sitting at a height, looking down over the nose and unable to see directly beneath you. The slant range means the first thing you see is a number of metres ahead of you.
Simply add the 90m to the bit you would be able to see if you could look straight down, and the baffling arithmetic works out fine.