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How likely it is to happen depends on how greatly the changes in this particular update affected your site's listing.
I hope what you're getting is good news.
At this time of the cycle it's probably for the reasons described above. It can also happen due to different preferences. For example, my Google default is to show 100 results, if you go to the same URL as me you might only see 10.
That's why it's a good idea to go to the link in the referer field and not to just type the keyword from the query.