Question regarding spirit level

[Augusti19]

Surely a dumb question but oh well...
Why does the spirit level show differently depending on whether it's upright or horizontal?
I need to use the spirit level in a horizontal position because it's cramped, but I don't want to set the thing completely skewed.
Upright = bubble upwards
Horizontal = bubble from the side

 

[C.Lundin]

The dragonfly is simply not the same that way. They are not in the shape of a rectangular block.