Here's the long answer how this works:

As per BIP39, the mnemonic seed is a multiple of 32 bits between 128 and 256 bit (128 bit entropy is the minimum to be considered safe), plus 1 bit checksum(sha256) for every 32 bit appended to the end of the entropy.

With 128 bits you have 4×32 + 4 bit checksum, meaning the 12th word contains 4 bits of checksum. 2^4 = 16 possible words, once we have the first 128 bit of entropy.

Additional info: the number of bits contained in the entropy + checksum always follows this formula Y * 32 + Y where Y= 4 or 5 or 6 or 7 or 8 (12, 15, 18, 21, 24-word mnemonic seed, respectively). This is always a multiple of 11, since 32Y + Y = 33Y = 11 * 3 * Y.

So it is handy for the bip39 words to encode 11 bits, and 2^11 = 2048 possible words on the list. So e.g. the 12th word in a 12-word mnemonic consists of 11 - 4 = 7 bits of entropy.

In the case of the SeedSigner, you can create your seed phrase by drawing random words from some container having all 2048 words, putting words back after a draw.

You draw 11 words, but you cannot draw the 12th because that only encodes 7 bits of entropy and the checksum is deterministic after that, not arbitrary. So after the 11th word it instructs you to perform 7 coin tosses (e.g. heads = 1, tails = 0) and enter the resulting bits into the UI. It will then calculate the last 4 bits of checksum and that gives you the 12th word.