# Why bits are mixed up (interleaved) during ATC transponder mode A and mode C reply format?

In the image above from radartutorial.eu, we can see that the sequence of bits are c1 a1 c2 a2 c4 a4 during the reply of ATC transponder mode A.

Why are the bits not transmitted like c1 c2 c4 then a1 a2 a4. Why this mixing up or interleaving?