This would be a good question for your builder.

We might be able to help if we you post a photo, otherwise ask your builder.

Thanks! Its been a year since handover :D, I didnt notice the depth difference.

I couldnt upload photos.

Now I could.

There are about 3 or 4 risers that are 0.8 m from the wall. On the plan they are surface drain. But i dont know whats the one closer to the wall. Thanks!

Out of millions of people in the world you'd think one of them should know what that riser is for.

I betcha if you check plumbing prints that was put there for ac condensate drain.

I think you are right. I now see a note in the plan