Why shouldn't I set the score for SPAM_99 and SPAM_999 higher?

[Thomas Cameron]

I understand that turning knobs without understanding the consequences 
can do bad thing, but almost all of the spam that gets through SA on my 
server has SPAM_99 or SPAM_999 set in the headers. It is obviously spam, 
so I don't really get how it wasn't flagged, but it wasn't. What are the 
risks of giving more weight to SPAM_99 and/or SPAM_999? Explain it like 
I'm five, sorry, it's probably something simple that I just don't 
understand.

Thomas