DDR3 Routing - Registered DIMM vs Unregistered DIMM

[shaiko]

Hello,

My question is about the following post:
https://www.bit-tech.net/hardware/memory/2008/02/10/the_secrets_of_pc_memory_part_4/3

At the bottom of the page you can see 2 examples of DDR3 routing.
Why should the routing for a Registered DIMM be any different from an Unregistered one?

 

[malli_1729]

Because Registered DIMM uses extra Buffer/Delay chip because of that Controller won't see much loading effect of the memories. Data transfers will be reliable.,

 

[shaiko]

Because Registered DIMM uses extra Buffer/Delay chip because of that Controller won't see much loading effect of the memories. Data transfers will be reliable.

This is all very good.
But WHY does it necessitate a different routing scheme than simple un-buffered DDR3 ?

 

[malli_1729]

Because REGISTERED DIMM involves extra chip --> So, traces will go to this Chip as input and output may be 2 sets will be transferred to different sides.