The short answer is there aree two zmanim for shkia. The regular sunset is the usual shkia, but there is another time later, just before the geonim nacht called the second shkiah, which is generally held of by chasidish communities. Until just after the war they used this time for kabolas shabbos as well.
It is well known that the Satmar Rov Zt”l and Reb Aharon Kotler Zt”l came to an agreement in the 50’s when they both lived in Williamsberg, where the chasidim would be mekabel shabbos at the first shkia, and the litvishe would keep rabeinu tam zman for moitzoei shabbos. This is why the yeshivishe crowd in the US generally holds RT zman after shabbos.