Google & Google Images indexing fluctuations

I've been monitoring the images on my website that are being indexed into Google images (ie. by searching site:www.mysite.com) and I'm noticing that the number seems to fluctuate randomly. It'll jump up to 550, then down to 65, back up to 496, back down. Anyone else have this happen to them?

While checking my grade at websitegrader.com, I also noticed that the number listed for pages indexed by Google (not Images, just regular Google) seems to bounce around too.

I'm wondering if this might be happening because Google finds more pages/images on my website than the "Google XML Sitemaps" plugin for Wordpress detects and notifies them of? ie. whenever I add another blog post, I believe it automatically generates and submits an XML sitemap (or pings Google to notify them)...could that be causing the fluctuations?

Just an FYI: You can submit the site maps yourself, via Google Web Master Tools. https://www.google.com​/webmasters/tools/home​?hl=en

Seems that Google images would just be a headache to monitor and contend for.

Consider how many more images must be put on and off the internet each day than there are single websites.

One person on Flickr could add a 100 images with tags. Another person can delete 2 entire albums. A website may add a gallery with 4 albums and 200 photos. The data and what Google does with it will dance all over the place.