What may not be clear from other comments is that the wallet that generates these seed words isn't actually selecting random words from the list. In reality it is generating a random entropy (a stream of bits) then encodes those bits to be human readable form which is represented by those words (so that you can easily write down words instead of bits).

Usually such encoding methods contain a checksum to quickly figure out mistakes if user entered wrong words in the future while recovering.

Each word represents 11 bits, and part of the last word is the checksum. So when you select random words, they are decoded to an entropy + checksum with the checksum having a high chance of being wrong.
12 words * 11 bits = 132 bits = 128 bit entropy + 4 bit checksum.