Self complementing Codes
"A code is said to be self-complementary if the code for 9’s
complement of N i.e. 9-N can be obtained by interchanging all 0s
and 1s." (This answers the why question)

Finding the complement of the digits:

9-2=7 , since 2 is 0101, then 7 is 1010
9-8=1 , since 8 is 1011, then 1 is 0100
9-3=6 , since 3 is 0110, then 6 is 1001


With this you know the digits 2,8,3,7,1,6 and can make any permutation with them you like.

As an addition, I believe if you don't know if the number is LSB first or MSB first, you can only know the complement of 8 (there are two options for the others).