What is sampling or precision?
Google Analytics (GA) restricts the amount of data it returns when a query includes a large number of dimensions or a large amount of data. In other words, if your data for a particular web property within a given date range exceeds a certain number of visits, GA will aggregate the results and return a random subset of that data, called sampled data. They do this to speed up their servers and reduce the data they have to hold in memory. Unfortunately this can lead to differences in numbers for different reports. This affects both the Google Analytics web interface as well as Grow since they use the same API.
Google will tell you when it is sampling your data, as you will see a yellow bar along with the sampling percentage and a learn more link.
Is sampling a problem? Sometimes. You can think of the sampling percentage as a rough confidence level estimator. At a 90% sample, you can be around 90% confident that the data is close to correct. At 50% we are 50% confident. At 1% sample, we are 1% confident. (Note this is a very rough estimate. Check and double check your data if you are going to be making important business decisions off of it)
There are several things you can do to reduce the amount of sampling that happens (from well written overview article)
Change Your Date Range
If you’re trying to look at a 3 year period, then you might want to consider changing that to a smaller time span, to get the total visits to your site under that 500k level. You can also create a report that is under 500k visits per month, then pull in a few reports with different dates and aggregate the data yourself in Grow.
Use Standard Reports
The standard reports in Google Analytics are never sampled. So if you are applying a segment, or using a custom report and get sampled, see if you can get the same data with a standard report.
Create New Profile with different filters
Maybe you want to look at the content reports for just your organic visitors, and applying the organic medium advanced segment forces you into a sample. You can create a new profile to just capture your organic traffic, and then put a filter on that profile to only allow organic traffic. If you apply any segments in the profile it’ll get sampled, but the standard reports will hold the unsampled information for just that organic traffic.