The best explanation for this behaviour is an example:

There are 2 products in the shop with the following properties:

A shop visitor uses the colour filter with multiple selection and is shown the number of items per filter value as follows:

By clicking the red filter value, the user receives 2 items as expected. If the Blue filter value is clicked again, the user would expect either 1 or 3 results based on the number (1) – but actually 2 products are displayed.

The number of filter values can also confuse the user, as this cannot be transferred directly to the number of final search results in the case of multiple selection. It is therefore unclear for the user how the selection of a filter will affect the number of results. Therefore, this value is hidden by default.