GA4 numbers -- like the volume of sessions for a specific landing page -- can be a little different in Oncrawl from you may see in Google Analytics.
This is due to how Google processes and provides analytics data.
What domain are we looking at?
First, make sure that the domain you are crawling and the domain that GA4 is providing data for are the same.
The way Google organizes properties makes it possible to create a single analytics property for multiple websites hosted on subdomains or even on different domains. Sometimes the differences between Oncrawl and GA4 are due to the fact that each platform is analyzing a different scope, a different set of pages.
If necessary, you can modify the scope of your GA4 data set to best reflect your website.
Sampling in GA4 data
When the problem isn't due to how GA4 was set up, differences are almost always related to sampling.
Any solution using Google Analytics (GA4) data faces the same issues, and you might have even encountered the same discrepancies within GA4 when working with explorations: Google samples data and limits collection rates.
In GA4, data in standard reports is pre-processed to make looking up data for your website faster. These reports are not sampled.
However, when Oncrawl requests information, it is similar to when you create explorations in the GA4 user interface. Oncrawl looks up data in un-processed tables. This process in GA4 is subject to sampling if the table is larger than the event quota.
You can find more information about sampling in GA4 here: [GA4] About data sampling - Analytics Help
In Oncrawl, the effects of sampling may be especially visible for high numbers (high volumes of traffic…).
Making the impact of sampling less important
If you use Google Analytics 360 on the GA4 properties you share with Oncrawl, you should see less sampling in your GA4 data, including when viewing it in Oncrawl.
As part of the service, Google Analytics 360 provides higher limits for Google Analytics 4 property data collection, reporting, retention, and export. They also allow for larger sample sizes, which can reduce or eliminate the effect of sampling in the data GA4 shares with Oncrawl.
Google has provided more details on this service: [GA4] Google Analytics 360 (Google Analytics 4 Properties) - Analytics Help