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.
It's been decades since you could do that for yourself. It must be something like 20 years ago on Alta Vista that I got burned using search queries as bookmarks and (surprise, surprise) suddenly not being able to find "that page" any more.