How Can MED affect a RR design?

In the video Brain quotes RFC 4456 which says

"both multi-exit descriminators (meds) and interior gateway protocol (igp) metrics may impact the BGP route selection... with certain route reflection topologies the route reflection approach may not yield the same route selection result as that of the full iBGP mesh approach".

I understand how IGP could affect this because in step 8 in the selection process it states that the best path is via the lowest IGP metric to the next hop. However, I do not understand how MED could affect the best path in a route reflector design because MED is always carried within an autonomous system. So it shouldn't matter how many routers advertise different MED values because the RR will still make the exact same decision as a RR client. So I do not understand why MED could lead to sub-optimal routing in a RR design. Can someone explain how this is possible?

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