Hey there,
Google has many datacenters all over the world. Depending on how often your webpages change, these datacenters may have different snapshots of your website, and therefore basing your rankings off different on-page factors.
The more often your website updates as well as other websites in your niche, the more erratic the search results are going to be across different datacenters.
If you want to improve rankings, work on getting quality content and quality backlinks to your quality content
Good luck.
Sincerely,
Travis Walters
admin@codebuyers.com