If you're on a shared server, other activity on the server can affect your site.
Even on some virtual private server, this can happen, too. You're guaranteed a minimum of resources, but when more resources are available, you can consume more.
Server side processes (backups, cron jobs, etc.) can affect measurements.
And if you have a good bit of traffic on your site, that can affect your measurements, too.
P3 has a good section in its help about is accuracy:
How accurate are these results?
The results have an inherent margin of error because of the nature of the tool and its multi-layered design. The plugin changes the environment to measure it, and that makes it impossible to get completely accurate results.
It gets really close, though! The "margin of error" on the Advanced Metrics page displays the discrepancy between the measured results (the time for your site's PHP code to completely run) and the expected results (sum of the plugins, core, theme, profile load times) to show you the plugin's accuracy.
If you want more accurate results, you'll need to resort to a different profiler like xdebug, but this will not break down results by plugin.
Try taking a few different measurements at different times.