Thanks, Guillaume! I just learned that this is due to a UX flaw in TSVB. Essentially, the last bucket is what's displayed in the non-chart visualizations, such as Gauge for example. So though you've picked "Today", you're not seeing all of the results from that entire time range; just the last bucket. This issue tracks the problem:
https://github.com/elastic/kibana/issues/13730.
Thanks,
CJ
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