I don’t think “typically” a lot more comprehensive" is a good description. Instead the alumni listed on the 2 types of Wikipedia pages appear to be typically nearly identical for the discussed LACs. For example, the Reed alumni page you linked above has ~340 persons, while the Reed category page has 329, suggesting that the alumni page is slightly more comprehensive than the category page in this example. The extra people on the alumni page and not the category page appear to primarily be alumni who do not have a dedicated Wikipedia page. The manually edited alumni pages can include names of “notable alumni” who do not have Wikipedia pages, while it appears that the category pages does not include such alumni.