There are several reasons why Google Analytics might be showing a lower number of transactions compared to your Magento store. Here are a few things you can check:
Make sure your Google Analytics code is installed correctly on your site. You can use Google Analytics Debugger to check if the code is firing correctly.
Verify that the transactions are being recorded in Google Analytics. You can check this by going to Conversions > Ecommerce > Overview in your Google Analytics account.
Check that your ecommerce tracking is enabled in Google Analytics. You can do this by going to Admin > View > Ecommerce Settings and ensuring that "Enable Ecommerce" and "Enable Enhanced Ecommerce Features" are both set to "On".
Ensure that the ecommerce data is being sent to Google Analytics correctly. You can use Google Tag Assistant to check if the ecommerce data is being sent correctly.
Check that the date range in Google Analytics includes the date range for the transactions you are comparing to your Magento store.
Verify that your Magento store and Google Analytics are using the same currency.
Make sure that the transactions you are comparing to your Google Analytics data are completed transactions and not pending or canceled orders.
I hope this helps you diagnose the issue.