Hello, this is Emily.
Considering the size difference, it definitely can be within normal range due to temp files, system restore points or shadow copies as part of its backup and recovery system, background processes, etc.
You can try running a disk cleanup to remove any unnecessary files and free up some space.
You can also download the Treesize app from Store App. Use it to identify which files or folders are consuming the most space on your disk. This can help you pinpoint the source of the fluctuations and decide if any action needs to be taken.