That would make sense if the other EC2 Xen hosts were running PV guests (as was required for the smaller / older EC2 hosts IIRC? ) & the cc1.4xlarge was PVHVM, according to: http://lists.xen.org/archives/html/xen-devel/2012-06/msg0102...

cc1.4xlarge was the first Linux HVM EC2 instance. All new families support HVM (and some are HVM only)

Should probably add a disclosure here. I ran the EC2 instance product management team for several years and continue to be involved.

Looks like a smoking gun then.