@Ondřej-Šutera Yes, 7 am on both. The discrepancy has now disappeared after the forecast was updated. I'm not going to worry about it.

@Ondřej-Šutera Yes, ECMWF

I withdraw my original question, as I discovered I was logged in to Windy Premium on Safari, but logged out on Firefox.

Then, I compared the forecast on my iPhone Windy app vs. Safari desktop, making sure I was logged in both places. There's still a difference in the forecast: see attached examples.

See attached examples. I'm using Windy Premium.

Location is Hilton Head Island, South Carolina, USA.