It has to do with difficulties in estimating carotenoid content of food. Certain assumptions are made about the efficiency of their conversion to retinol in humans.

National Institute of Health, USA guidelines:

IU retinol = 0.3 mcg RAE

• 1 IU supplemental beta-carotene = 0.3 mcg RAE
• 1 IU dietary beta-carotene = 0.05 mcg RAE
• 1 IU dietary alpha-carotene or beta-cryptoxanthin = 0.025 mcg RAE

RAE can only be directly converted into IUs if the sources of vitamin A are known. For example, the RDA of 900 mcg RAE for adolescent and adult men is equivalent to 3,000 IU if the food or supplement source is preformed vitamin A (retinol) or if the supplement source is beta-carotene. This RDA is also equivalent to 18,000 IU beta-carotene from food or to 36,000 IU alpha-carotene or beta-cryptoxanthin from food. Therefore, a mixed diet containing 900 mcg RAE provides between 3,000 and 36,000 IU vitamin A, depending on the foods consumed.