The biggest cause of discrepancies between two analytics system, is the users that leave before a page has finished, and one analytics system has loaded but the other has not yet. Discrepancies of a few percentage are reasonable. If there is a large discrepancy (like 10% or more) you need to investigate.
Ideally there should be no discrepancy between two analytics system on the same website. But they do happen for the aforementioned reason. There can also be different metrics, which may sound the same but are measured differently. As well as different configurations and cookie durations.
The reasons for discrepancies can include:
- Accidental clicks
- People hit stop before the page has loaded, or close the tab
- People load the page but the analytics didn’t load before they left
- Too many scripts loaded by the destination page
- A reasonable click discrepancy is >15%. If it is a lot higher you may want to adjust your campaign, either the settings, the creative or the destination page.
High discrepancies can be caused by:
- Slow page speed, make sure your page is fast to load.
- Mobile clicks, users tend to cancel more, experience slower network and/or leave before the page loads.
To help isolate the causes things you can do:
- Do a Google Pagespeed analysis on mobile and desktop. This can identify if slow page speed is causing a problem.
- In a spreadsheet, for the same duration, put traffic day by day, and by device. This should uncover if device is an issue or if ‘time zone setting’ is an issue.
- Evaluate where the scripts load on the page, do they load close to one another?