Different datasets, baselines, averaging windows, regions and update times. A difference of a few tenths of a degree between El Nino indices is normal.
Data and trust
Common reasons two numbers for "the same thing" differ:
Different datasets: ERSST (monthly ONI) vs OISST (weekly and daily Nino 3.4) vs MUR (map layer). They use different satellites and methods.
Different baselines: 1991-2020, a sliding 30-year period, 1980-2018... (see Climatology and the 1991-2020 baseline).
Different windows: a day, a week, a month, 3 months, 5 months (JMA).
Different regions: Nino 3 (JMA) vs Nino 3.4 (NOAA).
Different scales: NOAA SOI vs BoM SOI (about 10 times larger); Thai AQI vs US AQI.
Model vs measurement: a model estimate (CAMS, ERA5, forecasts) vs a station or buoy.
Timing: one source already includes last week, another does not yet.
Revisions: recent ONI and RONI values can change for up to two months.
When sources disagree, look at the direction they share (all warming? all dry?) rather than one exact number.