I have noticed my last week has been categorized as Threshold and the previous week as Unique, while both looks similar and more Base or Pyramidal
The categorisation relies on the ratios between the 3 zones. For something to be threshold its:
s1 < 4 * s2 && s2 > 0.5 * s3 (using Seiler zones, more or less)
So s2 needs to be more than half of s3. So your first example makes it but the 2nd does not (only just!).
These categories do need some work, especially for people who ride a lot (12h+). It tends to call everything base as you ramp up the hours which I don’t think is quite right.
These categorisations are based off a spreadsheet from @fastfitnesstips (Alex).