There are three limits and they exist for different reasons.

On our shared domains

Thirty new addresses a month on the bare shared domain, seventy five a month on your personal subdomain. Existing addresses keep working: the limit is on making new ones, not on having them.

The reason is reputation. Our domains are used by everybody here, and one account generating thousands of addresses is precisely the pattern that gets a domain blocked by the large mail providers. When that happens, every alias belonging to every user stops working at once.

Your personal subdomain gets the higher limit because it is less recognisable from the outside, so it carries less of that shared risk.

While your account is new

A new account can hold thirty aliases in total until it has gone thirty days without an abuse report against it. After that the total cap lifts and only the monthly one remains.

On a domain you own

No monthly limit at all. The reputation at stake is your own, so the limit that protects everybody else does not apply.

Adding a domain takes four DNS records, and addresses on it read as ordinary mail with nothing naming the service carrying them. See Adding a domain you own.

Custom names, specifically

Addresses where you typed the name yourself may be limited per day on some installations, while generated ones stay unlimited. A hand picked local part is more useful for targeted phishing than a random one, which is the only reason for the difference.