Thanks for the feedback. I meant to get back to sooner but the answer is long.
It's a valid question and one I agonised over when deciding all the Classifieds categories and facets (it was a massive task, the spreadsheet is mind-numbing). In the end, I decided not to have "Riding Style" filter for Hardtail.
There are two sides that need to be considered in any Classifieds category decision: 1) creating the advert, and 2) browsing/filtering for it.
Filtering your search is benefitted by having more filters but creating your advert only gets more complex as you increase the requirements. We try to strike a balance here. And in this case, I decided to leave out "Riding Style" filter for Hardtails to simplify the advert listing process.
The Bike Hub Classifieds has a wide range of sellers with massively differing levels of knowledge. It has to cater for complete newbies up to the most knowledgeable in the country all in one system. Unlike an eCommerce store or brand website, there isn't an employee creating every listing with expert knowledge (even then online stores still get it wrong). We don't know who might be making the advert. People get confused, they get it wrong, or they simple don't know things. Not everyone shares the same level of interested in the technical side of cycling.
So we try to design an advert creation form that is easy to complete (spoiler: there's going to big design change to the ad creation form in the next week or two that I hope everyone will appreciate - I'm looking forward to to the feedback on that once it's live). But this also means that sometimes there isn't all the information that the searcher might like. We always prefer to have the bike listed than not being listed. The thinking is that you can always enquire further if you're interested which is better than never having a chance to see it at all. It can be intimidating not knowing the answers to questions.
Anyway, this was my thinking when deciding to not filter "Riding Style" in the Hardtail category:
The Hardtail category has a high proportion of newbies/low end bikes, so I'd like the advert creation form to have fewer questions to avoid them getting overwhelmed and abandoning. We want Bike Hub to be friendly to newcomers rather than scaring them.
Following on point 1, less knowledgeable riders don't know the distinction between and XC hardtail and Trail/Enduro Hardatil. Don't believe me? Go look at all the Anthems and Epics listed as Trail/Enduro bikes in the Dual Sus category here.
With reference to point 2, it's more than likely that the filter will get flooded with bikes that don't fit the description and end up having limited use.
While trail hardtails are hugely awesome, they are not all that common. So leaving out a potentially confusing field for a low volume item is a justifiable omission.
Obviously, "Riding Style" is not the only consideration, there are many other facets that are considered for inclusion or omission from each category and sub-category. This is just the narrow reasoning for this case.
The reality is that with a public created content, there is always going to be some inaccuracies and less than ideal filters. Especially for the more niche and knowledgeable searchers.
We're hoping that with some thoughtful design and feedback from the community (like this thread), we can make it as easy to use and accurate as possible for all users in the future.