There are at least six reasons why search results may vary depending on when and where you search from. And that's just on Google, not talking about other search engines.
Usually the differences are small. The biggest variations come from geographical location, and that is something Google is doing a lot more about than it used to.
In any search, Google tries to figure out if you are looking for someone locally, even if you don't put in a location. So if you search for "plumbers" it can pretty well assume you are looking for a plumber in your area. On the other hand if your search phrase is "how to bake a cake" you probably aren't (as opposed to a search for "cake bakery").
Sometimes Google isn't sure and will serve up a mixture of local and worldwide results.
But how does Google know where you are?
Probably it doesn't know exactly where you are. But by your IP address (your Internet connection location code) it usually has a pretty good idea. For example, if you are on Roadrunner in Tampa Bay, it knows the IP address Roadrunner has assigned you (which you share with others). So Google certainly knows you are in Tampa Bay, and probably knows your location down to at least what city you are in.
It's all part of Google's ongoing effort to serve up exactly what you are looking for, and to make it as easy as possible on you. So that even a sloppy search is more and more likely to hit the nail on the head.
So-called "heat map" studies have demonstrated their success in this. An amazing percentage of the time, people now find what they are looking for in the first 1 or 2 results on the page.
And that's why Google remains the 600 pound gorilla of search, despite massive and expensive efforts on the part of competition.