I noticed that too. The 1940 census indicates the information on their household came from Doris (that x in a circle symbol means that - it was usually the wife of the head of the houshold).
In the 1930 census it is not clear who it comes from - but they do seem to be living in an apartment building of some kind. There are several households listed as being at 3028 Tracy Avenue - it is marked as the same dwelling but with several different families. Also, the record says Arch and Doris rented their home (for $55 a month).
I wouldn't really be surprised if another person in the apartment building, may have supplied the information. Perhaps Doris was out when the census enumerator stopped by, so he simply asked someone else for the information.
I notice that the info for Doris on her state of birth and that of her parents is entirely consistent with your other records. If the information was from the head's wife in the next apartment, I can easily imagine her having had enough conversations with Doris to know the where she was from, but perhaps not for Arch. As neither she nor Doris were born in Missouri, I can easily imagine the topic came up - especially with the likely noticable Alabama accent of the neighbor lady.
Of course, the previous two paragraphs are almost entirely speculation - reasonable, but speculative.