Gain on PPE as opposed to Gain on Investment Property

Hello,

I have a question regarding classification of gains.

A gain on PPE is taken to OCI whereas a gain on IP is taken to profit or loss.

I would like to know the reason for such classification.

Or is it just because of the rules of the standards?

A detail information regarding this topic would be helpful since I would be able to better understand the concept behind such classification.

Thank you.