I also noticed that traffic changes sometimes very unexpectedly. Then I looked at my crawlers log just for google and saw it pretty clear that as soon as I change something googlebot is beating my site to death :) For the month of April the lowest amount of heats per day by googlebot was 227 and the highest 823. In total I had 14.6K hits by gb for the whole month. I think that explains a lot when your average traffic raises twice and than you see that you've got 200 more pages indexed :) But it doesn't last for long time. I mesure my traffic in leads captured - if they go down from average week - I know there is something wrong, but even though I have my traffic increasing a lot sometimes - my leads are very steady.