Is the Ledger Implementation of BIP39 Generation Not Following the Spec?
Those are valid mnemonics with valid checksums. In  the bip39 spec is the following: 


  The following table describes the relation between the initial entropy length (ENT), the checksum length (CS) and the length of the generated mnemonic sentence (MS) in words.
  
  CS = ENT / 32
  
  MS = (ENT + CS) / 11


|  ENT  | CS | ENT+CS |  MS  |
+-------+----+--------+------+
|  128  |  4 |   132  |  12  |
|  160  |  5 |   165  |  15  |
|  192  |  6 |   198  |  18  |
|  224  |  7 |   231  |  21  |
|  256  |  8 |   264  |  24  |

As per the top row, 12 word mnemonics have 128 bits of entropy and 4 bits of checksum. That means any random 12 word mnemonic has a 1 in 16 chance to have a valid checksum.

The "blocking" of the valid final words instead of a totally random distribution is because the checksum bits are appended to the end of the entropy bits. So for every first 128 bits, there is exactly one valid final four bits. Bits are mapped to words by just counting down the alphabetically-sorted word list for each 11-bit chunk. So the first 128 bits determine the first eleven words and which "block" of the wordlist you are considering, and then within that block only one word corresponds to the correct checksum.