Hi peterlramos,
System Data can relate to third party applications or temporary files. You can find more from the information below and in the article here: Change Storage settings on Mac - Apple Support
"System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can’t manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac. You can manage your data that falls outside the other categories using the Finder or the third-party apps that created it."
When sorting by category with the steps here: Find and delete files on your Mac, do you see anything out of the ordinary?
Also were there any recent installs of third-party apps or programs that may be syncing or using temporary file space?
Keep us posted, and we'll continue to assist.
Cheers.