Because the site is wrong, simple as. As already stated, they havent taken into account many large cichlids in their profiles yet and even then, they probably only go on what google tells you, trust me, you see a full sized severum, you would never want to put it in anything less than a 4ft, 18" tank minimum. See the photo in my sig? That is a 5ft tank, see that large (not full grown yet) severum, now take two foot of that tank size and where would he swim?
I agree with Indigo, you should look at getting some of the Ellioti, you could then have some bottom dwellers and a good sized shoal or two of tetras, barbs etc. BP, you could have one, but I wouldnt add two, they can be aggressive and in that size tank, there is no room to hide.