The same ocean can be summarised every day, every week or every 3 months. Faster values react sooner but jump around; slower ones are steadier and are the official ones.
Indices
The app shows Nino 3.4 at three speeds:
Daily (Climate Reanalyzer, NOAA OISST v2.1): the newest picture, but noisy.
Weekly (NOAA CPC, OISST, 1991-2020 base): updated every Monday or so; the best "current" value.
Monthly 3-month means (ONI, RONI): the official record; about one month behind.
They will not match exactly: different datasets (OISST vs ERSST), different baselines, and different averaging windows. A difference of a few tenths of a degree is normal.
How to read it
Use the weekly value to see where things are heading, and the 3-month index to judge the official strength.