I experience the same thing and yeah it makes you wonder. I also find when a sponser has a "special hig payout promo" day and I pump in lots of extra traffic ratios will go to shit. Been burnt on that a lot.
Another thing I'm finding lately though is that sales seem to fluctuate madly at times. Feb and March for me have been all over the place. In March I've had my three highest sales days ever... and also had my three worst!
Case in point this is from one of my best sponsers that is normally fairly stable.
2007-03-14 1579 12 $420 1:131
2007-03-15 1517 0 $0 n/a
Exact same traffic. Join page hits shown.