IAS 27 separate financial statements

Sir as per IAS 27 we can measure investment in subsidiary in the individual FSs of the parent using any measure cost, IFRS 9 or equity method. When consolidating though, we remove any gains recorded in OCI or SPL if we used FVOCI or FVOL Model under IFRS 9 to measure carrying amount of investment in subsidiary.

Can you explain why? Understanding it will help me remember it better.