Those are two separate issues. Altavista's index certainly got updated over time, so changes in the search results aren't that surprising. I believe the issue

Those are two separate issues. Altavista's index certainly got updated over time, so changes in the search results aren't that surprising. I believe the issue OP is addressing is that even given an identical index, two different users might see to entirely different search results, as their individual search history is factored into the query.