Firstly, make sure you understand the difference between products, variants and items.
Products refer to Shopify products, which consist of one or multiple variants.
Items refer to items found in Merchant account, generally correspond to a single product variant.
Some reasons why number of made-available products differ from number of items:
Your products have multiple variants.
After feeds run, it takes up to 15 minutes for items to be updated in GMC.
Depends on the feed settings and product data, a Shopify product can generate none, one or multiple items.
Merchant account can also contain items of previous feeds from the past. To clear all items in Merchant accounts, login and delete all the feeds.
Unpublished products are not synchronized to GMC.
Other than above, items can be replicated by Google (automatically) to other countries that your shop is shipping to (more about this here). Replications are also counted into total number of items in GMC. This may not be desired in many cases:
You have a dedicated feed for the targeted country. For example you have a feed in GBP for United Kingdom and Google keeps replicated items from your USD feed to UK.
The number of items in your GMC goes up and hits the GMC limit (150.000 items by default).
