Buzz, your understanding is widespread and I believe it has been taught as being the correct way on many courses.
It seems that the way you describe is applicable only to multi-storey buildings with phased evacuation (see 20.2.C)
The A2 amendment (2008) of 13.2.1 (does anyone remember what this clause said before the amendment?) states that 'the manual call point on each level (in enclosed stairway), other than final exit level from the stairway, should be incoporated within the zone that serves the adjacent accomodation on that level'
I'm guessing that the importance of being able to operate an easily locatable manual call point is considered more important than being able to establish the probable initiation zone of a fire. Because, as things now stand your scenario of someone escaping a fire on zone 'x' may now easily operate a mcp connected to zone 'y' whilst escaping down a staircase.