Why so many false positives?

[#1]

I'm finding an increasing number of false positives in my SPAM folder. My filter settings are on the least harsh option.
Today I found a Newsletter from a bone fide source in the SPAM. The mail had passed both SPF and DKIM checks yet been marked as SPAM.
Whilst its a scoring mechanism and SPF and DKIM don't automatically mean HAM I'm surprised that a mail advertising new record releases from a known visible legitimate business scored as SPAM after passing both SPF and DKIM.
Are PN surprised too?

[#2]

Could you post (sanitised) headers from one or two of these false-positives?  It would give us a better idea of what might be causing the problem.
Thanks
Chris