It is recommended by the World Health Organization to consume at least 400-500 mg of calcium per day. This is to prevent osteoporosis. However, I found that on average, only 25% of the calcium is actually absorbed. Should people correct for this, so that you have to eat more than 2000 mg of calcium per day? Or does the World Health Organization (and many other sources who recommend certain amounts of calcium) take this into account?
They've taken into account absorption, which makes sense as these are guidelines meant for people to use to calibrate their diets. Reference (15) in the recommendation you link takes is:
Department of Health. Nutrition and bone health: with particular reference to calcium and vitamin D. Report of the Subgroup on Bone Health, Working Group on the Nutritional Status of the Population of the Committee on Medical Aspects of Food and Nutrition Policy. London, The Stationery Office, 1998 (Report on Health and Social Subjects, No. 49).
A copy is available online at https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/743523/Nutrition_and_Bone_Health_-with_particular_reference_to_calcium_and_vitamin_D__1998.pdf.
Quoting from that document:
The UK DRVs Jbr calcium set in 1991 (Table 4.1) were derived by calculating factorially from the needs for calcium for growth and for maintenance of bone mineralisation. Allowances were made for incomplete absorption and obligatory calcium losses