There's a difference between urine calcium and serum calcium. The supplements will artificially raise urine calcium and skew the test. You want to see urine calcium based on the normal diet. If that is high, then you have an issue. From another site, this is a good explanation:

"The reason to stop calcium supplements before a 24-hour urine calcium test is that the supplements will increase urine calcium levels. A study titled 'Do calcium supplements increase serum and urine calcium levels in post-menopausal women?' obtained the following results with calcium plus vitamin D:

'The serum calcium levels did not vary significantly in SG (study group) in spite of consuming calcium supplements over a period of time, whereas the urinary calcium levels increased progressively (p value < 0.005) in those who have taken calcium supplements for a year compared to those who have taken for a month.'

https://link.springer.com/article/10.1007/s12603-014-0532-2#:~:text=relevant%20statistical%20methods.-,Results,have%20taken%20for%20a%20month. "

njhornung, I see your point.
Suppose the use of the test isn't to discover the body's experience on a daily.
Generally the 24 hour urine is used to detect possible problems with the parathyroid gland or possible problems with the function of the kidneys.
An abnormal test result generates further testing.