The disk itself has no notion of what a filesytem may be. The disk knows what internal blocks are used and what are not through some metadata which never exit the disk, and the OS is completely unaware of them.
You are conflating two distinct notions of "free sectors", which occur on distinct machines at distinct levels. The disk itself works over a number of internal "blocks"; using these blocks, the disk maintains a logical view of the disk space as a long sequence of bytes (conventionally split into successive sectors). The disk will internally split the disk space over the blocks, with some redundancy, reordering, dynamic copies and other smart algorithms which together aim at spread wear so as to make failures happen less often; this is the "wear levelling".
The OS only sees the sectors, which are not necessarily of the same size as blocks (indeed, Flash blocks tend to be on the 16 to 64 kB size, while sectors are traditionally 512 bytes), and the wear levelling is hidden to the OS. The OS uses some sectors to encode the filesystem structures, and these structures mark other sectors as "free" or "in use". From the point of view of the OS, a sector is "free" if it does not contain anything interesting anymore (e.g. it was part of a file which has been deleted since); whereas from the point of view of the disk, a block is "free" if it was never written to, or if the wear levelling algorithm already copied its contents to other blocks, and the block can be internally reused.
If a sector has been written to, then the data made it into a SSD block; if the corresponding file is deleted, the sector is "free" from the point of view of the OS (the OS will allow itself to write to that sector the contents of another file), but for the SSD this still is data which has to be preserved -- and preserve it it does.
An enlightening (or not) feature to consider is the TRIM command. This is a SATA command by which the OS may inform the disk that a given range of sectors is considered "free" (by the OS), allowing the disk not to preserve their contents until they are next written to. When the OS has TRIMmed some sectors, reading the sectors directly (from the device, bypassing the filesystem layer) may theoretically return varying data bytes, depending on how the disk manages things internally. "TRIM" is the only way by which "free sectors" from the filesystem, and "unused blocks" from the physical medium, may actually relate to each other. They are otherwise wholly independent notions.