This made me wonder: is there some sort of implicit sexism at play here, and do some of these young female researchers feel the need to impress other researchers (both male and female) with their appearance?
It is certainly possible that this is because of some sexist problems in this specific society / community that are not present in the international community. However, given Occam's Razor, I think we should assume that the more simple and straightforward answer is that this is just a regional peculiarity of these meetings, and that we should not extrapolate too much from this without further evidence.
It is not at all uncommon for individual conferences or communities to develop their own standards and behavioral patterns. Basically, this is true for each event where students are likely to attend over multiple years, hence can learn from their experience in the year past. Assume, for instance, that you are a young female PhD student that attends such a meeting for the first time in casual wear, and you see many other students in a similar career phase dressing more formally. Would you not bring more formal clothing next time? I don't think that this should be qualified as "feeling the need to impress", or as any problem that needs addressing.
It is also not overly weird to see more senior people wear differently - they have been attending this meeting for a long time, and (a) don't care much about the current dress code culture, and (b) have probably been nurtured at a time when the dress code was completely different anyway. These things change over time.
Also keep in mind that your own expectation of what is "normal" wear in conferences has also likely not been set in stone from day 1, but is the product of what you have seen in "other conferences". There is no god-given law that jeans are more natural to wear to a conference than nice dresses, heels, or suits.
(Actually, in most non-science conferences, it is normal to dress rather formally, so one can argue that what you have seen in your conference is closer to "normal" than what you are used to)