It's so ODD that the regular train doesn't need a step up, but the
specially-designed airport train, meant for people with luggage, does.
Not that odd when you think about it. The airport trains are older, built in the 90s for when the airport opened. Low floor trains were far from common then. They have bought some newer ones with low floor sections but the older ones are still in use. The regional trains are newer, the first ones entered service in 2012 and they have replaced older trains from the 70s.
And I'm not sure the airport trains were meant for people with luggage. I wouldn't be surprised if they were built for business travellers carrying only a briefcase, and the thought was that people paying for the tickets themself would prefer the regular trains since they are 100 kr cheaper, despite the fact that they are 3-4 minutes slower.