Missing zones in embedded-stations

Unified API
1

On importing stations from /line/mode/{mode}/timetables/, I found that a number of stations embedded in that endpoint are missing zone numbers (under the .stations.zone key). For the tube and dlr modes, these are:

station_zones = {
    '9400ZZLUHPK2': '2',
    '9400ZZLUTCR4': '4',
    'HUBWWA': '4',
    '9400ZZLUBMY2': '2',
    '9400ZZLUCWR3': '2',
    '9400ZZLUKBN2': '2',
    '9400ZZLUSWK2': '1',
    '9400ZZLUBZP2': '2',
    '9400ZZLUHTD2': '2/3',
    '9400ZZLUASL2': '2',
    '9400ZZLUCGN2': '1',
    '9400ZZLUHRC2': '6',
    '9400ZZLUKNB2': '1',
    '9400ZZLULSQ4': '1',
    '9400ZZLUHPC2': '1'
}

I notice that apart from Woolwich Arsenal, there are all 9400..N NaptanRailAccessArea IDs rather than 940GZZ.. NaptanMetroStations - might be relevant? On that note, if anyone knows why NRAAs are given sometimes rather than the NMS, I’d be interested to know!