If you have selected EMER ELEC CONFIG during the checklist, IR3 remains powered whereas IR2 is only powered for 5 minutes and then becomes unpowered. When you later reselect the generators, IR2 is recovered but it won't provide valid data as it's not aligned. Also, when AC Bus 2 is re-powered, PFD2 and ND2 are recovered so the prior selection of FO on 3 ensures that the FO receives valid information from IR3.
I think that's it and I hope it helps.
STP