It's not uncommon that Growlytics's sessions and other numbers may differ from other vendors like Google Analytics, Mixpanel, etc. This article will help you understand how Growlytics tracks users and sessions.
TimeZone: Data ingested by Growlytics are timestamped in UTC but in report you see the data specified in timezone of your account (mentioned while setting upthe account).
Web bots & Scrapers: Growlytic's intelligent source identification mehanism makes sure bots and scrapers are ignored for session tracking. Bots like "Googlebot" or any other data scrapers are ignored. This can be the reason you are seeing the desripencies in number of sessions.
Google Analytics Discrepancies
Though Google Analytics and Growlytics track sessions similarly, the following table outlines factors that cause a discrepancy in your session numbers.
Time hits midnight
The current session ends at 11:59 PM and the new session starts at 12:00 AM.
Campaign source changes
New session begins even if it is within the 30-minute threshold.
Session Event Limit
After the first 10 events, tracking is limited to 1 event per second.