The API is deployed across multiple zones: ch-gva-2, ch-dk-2, de-fra-1, de-muc-1, at-vie-1, at-vie-2, bg-sof-1. When performing a compute call, you should use the relevant zone for your resource.
For example: https://api-de-fra-1.exoscale.com/v2/instance would return only the instances from de-fra-1, https://api-ch-gva-2.exoscale.com/v2/instance from ch-gva-2.
To obtain a list of all instances across all zones, you would need to do the corresponding request per zone.
Note that if no zone is specified during the call ch-gva-2 will be used by default.