My speculation is the ad valorem duty requires more manpower to implement and so that's why there's the specific duty option. Especially because they originally temporarily halted the de minimis changes due to USPS not being able to handle it.
Executive order 14266 is the most recent rates with 120% ad valorem or $100 / $200 specific (gated by date as noted above). [2]
[1] EO 14256: https://www.whitehouse.gov/presidential-actions/2025/04/furt...
[2] EO 14266: https://www.whitehouse.gov/presidential-actions/2025/04/modi...