There could be a number of reasons for this:
The “Action for out of stock (OOS) variants” option you selected maybe in action. The option allows you to include or exclude the out of stock variants. For example, if you choose to exclude OOS variants and all variants of a product (or the default variant of a single-variant product) are out of stock, that product will not appear in feed.
You may have chosen a variant option filter. For example, if you selected to include only Black variant of a shirt in feed, other products which do not have a Black variant will not be included.
Unpublished products are NOT included in feeds. Most of the time this is the reason why the number of products in feeds are less than the available products. Unpublished products are only visible to store users, therefore they need to be excluded from feeds.
