The difference in how they checked your status is the most informative thing in your post, and it isn't about the minimum at all.
Syndications are sold as private securities under exemptions from registration. One common exemption lets a sponsor take money only from people they already have a relationship with, and lets investors self-certify their accredited status with a checkbox. Another lets the sponsor advertise publicly, and in exchange requires the sponsor to take reasonable steps to verify that every investor really is accredited, which is why you got asked for a CPA letter. Neither one is safer than the other. The checkbox deal is probably not advertising, the verified deal probably is. The definition of accredited investor is set by regulators and has been expanded over time to include certain professional licenses alongside the income and net worth tests, so confirm the current standard rather than relying on the figures you have in your head, and a securities attorney or your CPA is who signs off on your own status.
Minimums are a sponsor's business decision. A $25,000 minimum means more investors for the same raise, which means more onboarding, more K-1s, more investor questions and a bigger admin cost per dollar. Some sponsors set the minimum high precisely to keep that count down. Some run two classes of units, where the smaller checks get a slightly lower pref and the large checks get better terms, so read whether you're being offered Class A or Class B and what separates them.
One thing worth planning around before you pick on minimum alone: your first year in a syndication also brings a K-1 and possibly a state filing you didn't have before, so the smallest check has the highest friction per dollar invested.