Here are some of the reasons why you see a difference in the number of products in the Merchant Center and Google Shopping Feed app
1. Google Shopping Feed app pushes your products along with the variants to the Merchant Center. So you notice that the number of products in the Merchant center is always higher than those in the feed app.
2. In case you have multiple countries, the products will be higher in number as Google Merchant Center displays all the products, including the different countries.
3. In case you notice the product number is lesser in the merchant center, it implies that some of your products have not been pushed to Google Merchant Center. If some of your products have any major data missing, such as title or description, those products are not pushed to the merchant center.