The advantage of running user IDs with 1000 (even if you have less than 9,000 identifiers) is that they will all have the same number of digits, so files, for example, with suffixes with UIDs will be sorted in numerical order automatically, even if the sorter uses only alphabetic numbering. And you don't need to fill in the numbers with leading zeros to get there.
Conversely, if you have only 1000 users, numbers starting with 1,000,000,000 look a little silly: 1,000,000,001, then 1,000,000,002, etc.
Therefore, for many purposes, it does not matter what you do. An unambiguous number of digits has some advantages, and therefore a value other than zero or one is often used as a starting point.
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