In brief
background-position places an image within its background positioning area. Percentage positioning aligns corresponding points of the image and area; center aligns their centres.
Useful to know
background-position places an image within its background positioning area. Percentage positioning aligns corresponding points of the image and area; center aligns their centres. [1]
When the image equals the area along one axis, changing the percentage there may have no visible effect. Check background-size before concluding that positioning is broken.
Sources
For further detail and to check the information: