The Russians supplied the export version of the Flanker to China. Naturally, Russian export versions do not contain the same level of sophistication in terms of avionics package as Russian Flankers.
That's not really true. The equipment level is practically between the Su-27S and the Su-27SK. Actually the SK had a more robust airframe and landing gear that upped its payloads. Initially, the Su-27SK sent to China were equipped with a downgraded Gardenia ECM pod, but even this ECM pod is the equivalent of what the Russians had built into their domestic only MiG-29s. Export MiG-29s don't have built in ECM. In any case it was also moot, since the standard more advanced Sorbitsya ECM pod was later shipped to China anyway. Furthermore, later the Chinese Su-27s and J-11s were tweaked to enable them to use the R-77, which was not an option for the Russian Su-27s until they are upgraded to the SM standard.
The Su-30MKK had an avionics fit superior to the RuAF Su-27s and still the MKK lost their matches.