What is a spamicity of exactly 0.5?

[Jason A. Smith]

All of the spam that I get with a lot of random words appended to the
end get a spamicity score of exactly 0.5.  Why is this happening and
what does that score mean?  I don't understand why they don't get scored
as spam since most are advertising the exact same website and come from
the same source.  Shouldn't those few known spam tokens outweigh the
random words?  Is there anything that I can do to improve bogofilter's
detection of spam like that with random words?

~Jason