Why is Shia Maghrib after sunset?
Most prayer apps put Maghrib at sunset. In Ja'fari practice it comes a little later: once the redness that rises in the eastern sky after sunset has passed overhead.
Sunset is not the sign
At sunset the sun's disc drops below the western horizon. At the same time a band of red light appears low in the east, opposite it. As the sun sinks further, that redness climbs, crosses overhead and fades toward the west.
The Ja'fari school takes that moment, not the disc touching the horizon, as the sign that the sun has truly set. It is the more widely held view among Shia jurists. Ayatollah al-Sistani, for example, rules as an obligatory precaution that Maghrib is not prayed before the eastern redness has passed overhead. The same moment ends the fast in Ramadan.
How much later
The redness passes overhead when the sun is about 4° below the horizon. It gets there about 13 minutes after sunset near the equator, 15 to 20 in Iraq, Iran or Pakistan, 20 to 30 in London, and close to an hour in Oslo in June.
A timetable that puts Maghrib at sunset is early by that much, every day, and nothing on it says so.
How the app calculates it
Shia Namaz finds Maghrib from the sun's angle below the horizon: 4° with the Qom method, 4.5° with Tehran. Sunset stays on the timetable as its own, lighter row, so you can see the gap.
If your mosque or marja' gives different times, follow them. You can enter their angles under Custom.
See today's Maghrib for your city →
Source: Office of Ayatollah al-Sistani, Islamic Laws, times of Maghrib and Isha. Times above are from the app's engine with the Qom method; Tehran adds a few minutes.