I would like to buy a ZVV ticket for zone 121 only.
The minimum for ZVV zone tickets are 2 zones or tarif level 2. This means the ZVV has no tickets for a single zone. The exceptions are the city zones of Zurich and Winterthur (zones 110 and 120), as these count double and therefore are equal to two zones.
However, zone 121 counts as only one zone. As you have paid for tarif level 2 (1-2 zones), you receive a second zone counting as one. This is useful, for example, if you need to calculate the amount of zones you require for an additional ticket (e.g. when upgrading for a ticket valid for all zones).
An example: You receive a ZVV ticket for zones 121 and 122 at the ticket machine in Wallisellen for a direct trip with Glattalbahn from Wallisellen to Zurich Airport. This is the case even though the entire journey takes place only in zone 121 and zone 122 is not required for this connection. As you have paid the tarif for 2 zones, you will receive a ticket valid for two zones.