All the different data sets have different base periods for the anomalies. (realclimate.org)
During the course of the year, CPC introduced an ONI dataset using different base periods for determining anomalies for each year, with the most recent years (1995 to date) utilizing the 1981 — 2010 base period. (skepticalscience.com)
(Please note that different countries report anomalies with respect to different base periods. (ncdc.noaa.gov)