The Cache Manager isn't anything to do with the browser cache, this is a cache designed for use specifically with WebVT®.
When you navigate to an hour on the log then in the background low-quality files are downloaded and held within your browser so they can be accessed instantly.
Cached files are monitored in case a newer version of a downloaded file is available compared to what's already been downloaded. If the file is older than 24 hours then it is removed from the cache.
You can clear the cache yourself if you wish. To access the cache just click Cache from the Launcher.
On this pop-up, you will see a list of all the cached files denoted by their UIDs, their sizes and when they were last changed.
Clearing the Cache
To clear items from the cache, you choose the ones to delete by checking the tick box on each line you want to remove. There is a checkbox at the very top of this column which toggles all the items on the list between being checked and not checked.
Once you've selected the items you want to remove, just click Delete.
Once a cache is cleared you will find the log does re-download the items it needs. This is normal.