There are two different customs women follow when lighting Yom Tov candles. Many first say the bracha and then light, as opposed to on Shabbos where they first light and then say the bracha. The logic behind the switch is as follows: the bracha on lighting constitutes a kabbalah of shabbos/yom tov – on shabbos, saying the bracha first would mean shabbos has started and the act of lighting would be prohibited, so we light first and then say the bracha; on yom tov, lighting a candle is permitted, so there is no problem in saying the bracha first (in fact, there is the added benefit of it now being over l’asiyasan) and then lighting. Others do not reverse the order because lo plug – the custom established saying the bracha after lighting as a universal rule, and we avoid creating any differentiation between Shabbos and Yom Tov.
There is an additional difference in customs as to when to light the Yom Tov candles. Some women light a number of minutes before shkiya, as is done on Shabbos. Others follow the custom of lighting after dark before the meal – since lighting a candle on Yom Tov is permitted, there is no reason to push the lighting back to before sundown like we do on Shabbos.
The Shmiras Shabbos k’Hilchisa points out that one should not err and adopt mutually incompatible customs. The whole reason for saying the bracha after the lighting is to not differentiate Yom Tov from Shabbos. Yet, lighting after dark by definition is possible only on Yom Tov and not on Shabbos, a clear differentiation between the two. It would make no sense to light after dark but to only say the bracha after lighting.