Very observant!   The answer is that the monthly data you are quoting comes from the OISST dataset.  We calculate the ONI using ERSST.  Indices from both datasets are available on this webpage (link).  Tom recently wrote a blog post explaining why there are different SST datasets, which you might find to be helpful:

https://www.climate.gov/news-features/blogs/enso/exactly-same-completely-different-why-we-have-so-many-different-ways