There has been some speculation recently that searches are no longer isolated from each other in Google but that the previous search that you made may influence the current one, even when you aren’t logged in to any form of account. This follows an interview by Danny Sullivan with Google’s Marissa Mayer in April. This type of linked search has been used in providing targeted Adwords adverts for a while but it has never previously affected natural results.
Just how much of a difference this might make to results is impossible to predict as yet but it seems that it’s likely to mean that ‘standard’ results are less and less standard. Some webmasters are linking this to the recent strong fluctuations in rankings that have been seen – where positions have been markedly different within hours and sometimes minutes of queries being made – though personally I would be surprised if this were the cause of such major differences. That felt much more like a reindex or algorithm tweak, whereas I would expect the “previous query” effect to be either a relatively subtle refinement or alternatively a much more obvious shift of results where the earlier keyword has impinged on the later results.
It does however have other implications for SEOs reporting to clients. We’ve become used to clients seeing slightly different results due to connecting to different datacentres; if they now start seeing different results depending on a previous query then explaining such varying rankings is going to take even more time and persuasion