How do you know?

Health > Heart Rate Variability > Show All Data lets you see the exact timestamp and source for any stored data point. And for HRV specifically it will give you the list of millisecond-timestamped heartbeats it considered for that measurement as well.

But how do you know that this is (more) accurate?

Because HRV is the most reliable after parasympathetic activation in a nutshell. You can look it up for a better explanation.