There is no mistery there:)
Different devices with different screens sizes and different proportions are visiting your store. For each different screen, different size of image thumbnails are required, so they are being generated and stored into the /images/thumbnails/[width]/[height]/ folder.
So once they are generated, they are available via a direct link, regardless of whether the lazy_thumbnails tweak is enabled or not - they are already generated and available. In case when thumbnails have not been generated yet (or thumbnails were cleared), their generation will take place:
- Along with the page generation, increasing total time spent on page generation, if lazy_thumbnails tweak is disabled.
- Separately, once the page is displayed to the customer of your store, thumbnails will be generated on the fly, if lazy_thumbnails tweak is enabled.
As I have already mentioned in this topic Redirecting missing images to index.php instead of page **404** - #2 by CS-Cart_team, you need to contact your server administrators / hosting support and ask them to set up your web server that way, that it will forward the incoming requests to the missing images, directly to the CS-Cart index.php script.