Why did ShortPixel Image Optimizer spend more credits than what it told me?

If you log in to your WordPress site and navigate to the Media > Bulk ShortPixel page, you will see a number of images not yet optimized by ShortPixel Image Optimizer (SPIO). As seen in the next screenshot, that number includes images and thumbnails and it is only an estimation.

This is how the estimate is displayed in SPIO 5 versions and beyond.

This is how the estimate is displayed prior to SPIO 5 update.

In a few specific cases, this number may not be accurate and you will find that ShortPixel Image Optimizer ended up spending more credits than the initial estimation:

• If the WordPress theme is changed and a thumbnail regeneration isn't properly done (with reGenerate Thumbnails Advanced) on the whole Media Library, then older images could store many more thumbnails in the database (and on disk) from the previous theme, which will be optimized by SPIO.
• If during the bulk optimization something happens, the final number of optimized images will be different. This includes things like activating a plugin which generates new thumbnails, or a plugin that could regenerate certain thumbnails (such as WooCommerce), changing the active theme or adding new images.
• The "unlisted thumbnails" (optimized with the Optimize other thumbnails option located under Settings > ShortPixel > Advanced) are not counted in the estimation. That's because doing so would take a toll on the performance of your website.