Look up the debates on the bill. It could be anything. Maybe they were simply retained from the 1857 tariff. Maybe they were hoping to gain warehousing revenues from goods with a final destination elsewhere. Maybe they were hoping that some idiot would come along and try to import things that he could get for half the price at home without the tariff.
Then how come they had zero tariffs on things they didn't grow or manufacture at home, like coffee? If it was simply a revenue tariff, that's where you sock-it to them. They taxed canned fish at 15%, but not coffee?
They were protectionist --- they wanted to protect their citizens from competition. Even though they had much less than the North in the way of manufacturing, in 1860, they had the fourth largest manufacturing base in the world --- behind England, France, and the Northern states. They wanted to protect not just their sugar, cotton and tobacco farmers from competition, but also their fledgling iron, textile and metal working industries.