Same exact thing with EU-WEST-1/2.

  $ dig ns eu-west-1.amazonaws.com +short
  ns3.p31.dynect.net.
  ns1.p31.dynect.net.
  ns4.p31.dynect.net.
  ns2.p31.dynect.net.
  
  $ dig ns eu-west-2.amazonaws.com +short
  u6.amazonaws.com.
  u5.amazonaws.com.
  u2.amazonaws.com.
  u4.amazonaws.com.
  u1.amazonaws.com.
  u3.amazonaws.com.
  pdns1.ultradns.net.
  pdns3.ultradns.org.
  pdns5.ultradns.info.
  ns2.p31.dynect.net.
  ns1.p31.dynect.net.
  ns4.p31.dynect.net.
  ns3.p31.dynect.net.

I wonder why this is, considering the more extended usage that -1 on each region usually gets. :S