Hi,
When you create filters via the menu Display>Filters, you can select the type of display (dropdown, checkbox, radio, list, etc).
You can also select whether you want it to sort the data with the "apply on" option, or use the values of the fields/characteristics/categories/etc
With all the options available on that screen, you can make pretty much any type of filter which are used by big ecommerce websites.
The results are never random. However, it might seems random because you don't see the logic behind. For example, regarding the prices, due to technical limitations on calculations with current servers, it is not possible to dynamically calculate the discounts on the prices for the filters. Which means that either you have to use a precalculation mechanism which is quite complex to do or filter the products based on the undiscounted prices.
For now, HikaShop uses the undiscounted prices. And thus, if you have prices with discounts, you might get strange results regarding the filtering by price since it is the undiscounted prices which are used and not the discounted prices.
Regarding the filter module, it will display only the filters that you selected in its options.
So if you configured it once with one filter and that you added later on additional filters, you won't see them by default in the filter module as you will have to select them too in the options of your filter module.
I suppose that it must be why you don't see your new filters in the module so please check the options of the module.