uuidgen has consistent 13th digit and it's 4 I ran uuidgen 10k times and noticed that the 13th digit is always 4.
Why is that?
You can find a full explanation in the Wikipedia article on UUID.

UUID's come in different versions. The 4 you highlighted indicates version 4 was used where the UUID is created by generating a random number. Other versions use for example the date-time and MAC address of your computer.

When you look further at the first character of the next block you will see it is not very random either: it shows only the characters 8, 9, A and B . This is because the two most significant bits are 01 for the variant of this UUID version.