Often traditional analytics have been blocked by ad blockers and/or use the data in a way that consumers have chosen to block them.
That means, often when you swap analytics, your total traffic goes up. Why? Because you are now measuring more of your audience.
The second reason is speed. Mobile users and users that leave the page quickly may not be counted by traditional analytics which load slower. Newer analytics which load faster, end up counting more of your visitors.
In total could be between 10-15% of your current traffic. Or if you have a very technical audience, the numbers could be even higher.