Quick question to understand AD replication between sites/DCs

I just inherited a network setup and I am trying to decipher why decisions were made in the past.

I have three sites with four DCs:

Boston (1 DC) ----- Chicago (2 DCs) ------- Phoenix (1 DC)

Chicago is the primary location for all things with Bos and Phx being branches.

When I get into Connection properties of the NTDS Settings of each DC I am getting mixed information. For example one of the DCs in Chicago replicates from all of the other DCs but only replicates to the other DC in Chicago and the one in Boston.

I look at the other properties of the DC in Chicago and it only replicates from the DC in Chicago and it replicates to the other Chicago DC and the Phoenix DC. Why not the Boston DC?

It is scatter-brained like this so I am trying to better understand the reason why it may have been set up like this. I have seen latency when creating A records or other objects between the DCs and this has caused some lockout issues.

Thanks,

- Gymmbo