non-controlling interest in consolidated statement of financial position

in one question to work out the non-controlling interest in the consolidated SOFP it uses:

fair value of non-controlling interest at the acquisition date
plus NCI% x (retained earnings of subsidiary in reported date – retained earnings of subsidiary at date of acquisition)

however in another example it uses PURP and subtracts the NCI share of unrealised profit:

fair value of non-controlling interest at the acquisition date
+ NCI% x post-acquisition profit
– NCI% x unrealised profit

Q> when is it that you use the PURP, and when is it that you just use the retained earnings at reporting date – date of acquisition, and why are they different?