A RAM register on the flash controller records the erase count of all blocks to identify which ones are frequently or seldom used. A flash cell is made up of pages, and several pages make up a block.
This gets worse with higher capacity devices. With an SSD without integrated encryption, this command will put the drive back to its original out-of-box state.
If the SD cards stores a root file system, it is beneficial if the controller of the card can cache the segment s where the write operation s takes place, the segments, which store the metadata for the file system and if available the journal of the file system.
If the user saves data consuming only half of the total user capacity of the drive, the other half of the user capacity will look like additional over-provisioning as long as the TRIM command is supported in the system.
If the host wants to write new data to a used block, pages containing valid data have to be copied to an empty block, and the previous block has to be completely erased in order for that block to be usable again. SanDisk has a forum.
It will take a number of passes of writing data and garbage collecting before those spaces are consolidated to show improved performance.