Hi,
Counting accurately the products of a category can be quite complex as it needs to handle sub categories, sub sub categories, products in several sub categories, plugins, and potentially many of the settings in the menu item options.
For example, if you activate the "show discounted products only" setting of your menu item, the products without discount won't appear on the products listings of the categories of your menu item. However, when the menu item is displaying the categories listing, it doesn't take into account this setting and count the products whether they have discounts or not. If we wanted to take this setting into account, for a categories listing with 10 categories, we would need to run at least 30 complex MySQL queries (3 for each category) in order to know whether we can count or not the products since knowing if a discount can be applied to a product or not is actually quite complex in itself.
So, to make it easier for us, and also to lighten the processing of categories listings (which would be quite problematic if the count of products is activated and you have tens of thousands of products, and would probably just crash in the middle) we decided to not support that "show discounted products only" setting.
Another example would be if you have the
www.hikashop.com/marketplace/product/222...-product-plugin.html
plugin on your website. This plugin will filter out the products from the products listing if their sale end date is in the past or their sale start date is in the future. However, on the categories listing, the plugin doesn't do anything and thus the code counting the products is not affected by it.
So basically, what this all means is that you probably have a setting in your menu item which restricts the number of products being displayed on the listing, or that you have some kind of plugin filtering the products, or something like this, which would be complex to support for the counting of the products on the categories.
A simple solution in that case would be to turn off the filtering you're using on the products listing so that the products listed match with the counting.
Not knowing how you setup your menu item, nor what plugins you're using, I can't say for sure what is filtering the products though.