Google Ads API gives radically different forecast metrics from those seen in the web UI

We do this sort of thing: https://developers.google.com/google-ads/api/docs/samples/generate-forecast-metrics

and the results we get are radically different from what we see using the Google Ads web UI, given precisely the same parameters. The API call always gives a lower number of clicks, by a factor of up to about 2x.

Also: We don't see any of the KeywordPlans, KeywordPlanAdGroups, or KeywordPlanCampaigns that we create programmatically in the UI. This is a real pity, as it would really help to debug things.