Tag history interpolation - not returning interpolated values

Changing from Periodic | 1 hour to Periodic, 1 minute gives interesting results.

Now we can see that the first reading at midnight is lower than the first reading on the as-stored table as would be expected if interpolation is happening. Similarly the 00:10:00 reading is a little higher than the 00:09:54 reading as would be expected.

What’s going on here? Why does a 1 minute period seem to interpolate but a 1 hour period doesn’t?