How does a capacitor and resistor affect an SCR's firing angle?
You are assuming that the voltage across the capacitor will be &quot;in phase&quot; with the sinewave voltage applied across the circuit (AC source). That isn't true.
The true waveform across the capacitor is another sinewave of lower amplitude but, importantly, it will lag the AC source voltage by anything up to 90 degrees: -

Trace A (blue) I'm using to represent an applied voltage (AC source) and the green trace (Trace B) will be the resulting voltage across the capacitor for specific values of R and C. As you can see the peak capacitor voltage occurs way after the peak of the applied voltage and if the SCR trigger threshold were set about 0.7, the SCR would trigger here: -

As you can see the trigger point occurs after the voltage peak.