Rails - Jun 2014 - SerializationTypeMismatch when retrieving (not: storing) object

I don't understand the following example from the ActiveRecord::Base
section in http://api.rubyonrails.org/ :

=====class User < ActiveRecord::Base
  serialize :preferences, Hash
end

user = User.create(preferences: %w( one two three ))
User.find(user.id).preferences    # raises SerializationTypeMismatch
=====
What was the design choice, that I get the exception in the *last* line?
I could understand, if there is an exception in User.create, because
:preferences is supposed to be a Hash (if I understood correctly the
'serialize' function), and instead an Array is passed. But the
documentation clearly says that the exception happens at retrieval:

=====You can also specify a class option as the second parameter that'll
raise an exception if a serialized object is retrieved as a descendant
of a class not in the hierarchy.
=====
Is there a deeper reason, why this error is caught at retrieval, and not
already at the time of storing the value?

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