I'm thinking that Larsson has two standard s's because they come from two separate syllables and just happen to appear together in the word - Lars - son. I don't know whether this is the same for the other name because I don't know its derivation. I have in my notes on the older handwriting that the s's are written separately in the middle of a "compound word". And then in Voss and Priess they are proper "double-s" occurrences so are written like that.
Hopefully Elisabeth will give us a proper answer though, because the above is just my guess! And I don't know whether they should be keyed as ss or ß so hopefully Elisabeth will know.
(But Voss and Grosse are definitely actual surnames even if they aren't in the dropdown menu.)