What causes the discrepancy in full immunity between the bivalent and trivalent OPV? 

The trivalent OPV was first introduced in the global effort to eradicate polio. However, in 2016, after the eradication of wild poliovirus type 2 in 2015, the world moved to the bivalent OPV. Many countries are now using a combination of the bivalent OPV and IPV to address any gaps in immunity.