That's a combination of old information and Disney trying to discourage guests from arriving too early. The terminal can handle the capacity of the Magic and Wonder pretty well, but the Dream and Fantasy have so many more guests that it was super crowded with everyone arriving at 10:30 hoping to be the first onboard. So they added the arrival times in hopes of spreading out the guests arrivals. Once boarding starts it's not a problem, but before it begins with everyone flooding in with nowhere to go, it's crowded!
There was a time when the earliest arrival time was 11, and they were strict about not letting anyone in until 11. Even the local hotels where shuttles used to start around 10 or earlier, changed their shuttle times to not even begin running until 11 or later. Disney was turning away cars if they arrived too early.
When Disney added the 10:30 arrival times, they didn't really update the port opening information. They are still doing everything they can to spread out arrival times. Many cruisers pick a 12:30 or later arrival time because that's all that's left and really believe they can't arrive until 12:30. Disney isn't really discouraging that notion. If someone had a late arrival time and got there really early and the terminal was full, they'd have to wait outside a few minutes until guests inside boarded and made some room. That's pretty rare though.
As Amy and Kris noted, it's not a problem to get into the terminal at 10:30. The guests arriving on the shuttles from the airport will get there around 10:15.
In practice, the arrival time you picked will really just determine how high of a boarding number you have, and with an arrival time of 10:30, you'll get an early boarding number no matter what time you actually arrive.