I just played around with this, trying to make it happen, and I see several possibilities. This based on not a great amount of testing, but with some prior observation about Instant's quirks....
One possibility is that once I get to a "branching point" (eg, I type a space after a word), Instant seems to want to display the next most likely result in the chain, but if I haven't actually selected that choice, say by cursoring down, then when I hit enter I get the result-set that was displayed before the branch point.
The other possibility is that if I pause before the branching point... ie, before I type the space... sometimes, if I pause long enough, the result set will change by itself.
To illustrate the above with a hypothetical example... if I were typing [red widgets], eg, and I stop with the "s" in widgets and hit enter right away, I get what I was looking at when I'd gotten to the "s". If I type the space, then the results display switches to the results for the #2 suggestion... say [red widgets for windows]. But if I hit enter before cursoring down to select the second choice, then I end up with the [red widgets] results set. So, that might look like the results have changed.
I've also seen a results set switch for the same query if I just pause at [red widgets], before typing the space. I don't have enough time right now to try all the prior conditions to see what might cause that kind of switch, but it doesn't seem to revert back to the first set if I hit enter after I see it.
I can imagine, though, that if results were in the process of switching just as I hit enter, I might think hitting enter had caused the switch.
Can you describe (examplifying your search terms) what you're seeing.