2sheds you're putting words into my mouth. The book says "
indicating above/below/ etc." for the same reason you say
indicating XXXXft in your traffic information.
As nicevectoring points out, the altimeter encoder encodes and transponds based on 1013.2, and any local conversions are done on your radar based on your local settings and Transition Altitude, so irrespective of the pilot's setting, the readings will be equivalent for all contacts.
This shows when a pilot hasn't reset his sub-scale, and is flying (typically) an altitude with (typically) 1013 set and is a few hundred feet adrift from his stated level. A technical bonus which I have used a few times to some pilots' advantage to prevent their further embarrassment
Also to re-state Widger's point, this phraseology doesn't date from heightfinding equipment, but from the mid-air at (Delhi??) when the quoted level was taken as a level to descend to and - surprise, surprise - they met. The ICAO guidance thereafter was to refer to relative levels (known traffic) or no level at all (unknown traffic).
Tori