Forecast.solar doesn't seem to agree with the weather forecast

Can anyone help me understand why forecast.solar is giving me weird predictions?

The weather for today is rainy. The weather for tomorrow is sunny. Yet forecast.solar is giving me near identical predictions

It’s showing me 15kWh today and 17 tomorrow, whereas solcast is showing 5/25 (and solcast has been fairly close historically)

I’ve checked via their API and it even shows me my correct address, so I’ve definitely got the lat/long and azi/Dec/kwp values set correctly.

Solcast.api seems to get the prediction correct!