I think it is highly unlikely it would be detrimental. Firms are aware that at the moment of entry in the profession candidates can have interests in a wide variety of practice areas; and no firm is the best in every single one. As such, you expressing interest in a practice area where there are other firms which are somewhat better ranked is no issue. I think it could have only really been a problem either if the practice area was one the firm did not offer at all, or if the practice was a small/niche one in the firm.
Fundamentally, to have a good 'why the firm' reasoning, I do not think you necessarily need to show that
Ashurst is the best firm for every single one of your relevant selection criteria (ie practice area/sector interests, training style, strategy etc). You only need to provide a convincing argument that, all things considered,
Ashurst is the best firm for you.
As such, imagine you provide three reasons for wanting to join
Ashurst: (1) a strong corporate M&A practice, (2) leading expertise in sectors you are interested in - say, financial services and oil & gas; and (3) leading real estate practice. Now, if your only motivation was centered around M&A, this would be an issue, as firms like
Freshfields and
Slaughter and May are arguably better for it. However, these two firms will likely not be equally strong for the sectors/practices at (2) and (3). Thus, if you care about all three, it makes sense that you would pick
Ashurst.