When your content changes, so do your ads.

The technology behind Google AdSense goes beyond simple keyword or
category matching. In essence, they try to understand your content and deliver
ads that are relevant to specific pages, automatically, no matter how many
thousands of pages your site may have, or how specialized or broad your
content. With that being the case, as your content changes, Google's ads change
to match.
What that means to you is that if you go changing your content too much; do not
be surprised if the ads change as well.
And while many people complain about Google, they do offer some very useful
safeguards which you may want to explore.
• Competitive ad filters. You can block competitive ads, or other ads you
want to keep off your site, simply by telling them which URLs to block.
• Ad Review. Before ads appear on your site, they're reviewed using a
combination of human and automated processes. The review process
takes into account a variety of factors, including the quality of the ad and
whether it's suitable for all audiences.
• Sensitive content filters. At times, certain ads may be inappropriate for
some pages. For example, Google automatically filters out ads that would
be inappropriate on a news page about a catastrophic event.
• Choose your own default ads. In the unlikely event that Google is unable
to serve targeted ads on your page, they offer you the option of displaying
a default ad of your choice. This ensures that your advertising space is
always being used as effectively as possible.