That isn't my understanding, but maybe there is something I am not aware of.
From their help page -
The search results and ads appear as they would on Google, except the links on the preview page aren't clickable.
My understanding is the results provided there could be from one datacenter or another based on the user session.
Being logged in or out - personalized search - isn't so much the issue as is google has multiple datacenters. Each datacenter may not have the exact same results as all others. Dependiing on your user session, you could hit one of many different datacenters - and thus receive different rankings/results for the same search. Over time the rankings are propagated to each datacenter, so eventually when you do search the rankings will be consistent. And the beat goes on ... if you watch so closely.