crm issue in The DataWareHouse Toolkit 2nd edition

hi,
I'm a newcomer in the world of BI and I have recently started to read The Datawarehouse Toolkit : a guide to dimensional modelling (2nd Edition).
In Chapter 6, page 158, there is something that puzzles me.
A dimensional model for a CRM-system is diagrammed as follows :

Table FactTable ( ContactKey FK, ExtendedCustomerKey FK, more foreign keys ..... )
Table ContactDimension ( Contact Key PK, ..........., ExtendeCustomerKey FK )
Table ExtendedCustomerDimension ( ExtendedCustomerKey PK, ............... )

To me it seems this example is modelled wrong.
What puzzles me is the foreign key in the table ContactDimension which points to the ExtendedCustomerDimension table.
Why is it there ?

Personally, i would follow it as follows :
FactTable ( ContactKey FK, ExtendedCustomerKey FK, ......)
ContactDimension ( ContactKey PK FK, .......)
ExtendedCustomerDimension ( ExtendedCustomerKey PK, ........)

By setting a foreign key constraint on the primary key of ContactDimension I would create a 1 on 0-1 relation.
So 1 row in ContactDimension equals 1 particular row in ExtendedCustomerDimension but not the other way around.

Is there any reason why Ralph Kimball opts to not model this example in this way ?
Because now he uses a plain old Foreign Key which renders the relation a 1 on many relation.