Why Does Relative Magnitude of AC Current Waveform Decrease Based on Phase Ange?
Brian Drummond's comment about light bulb resistance increasing with temperature is (most of) the answer. The cold resistance of an incandescent light bulb filament is typically 10 times less than at its rated voltage. At low phase angle the power drawn by the bulb is less so it heats up less and draws more current than it would if its resistance remained the same at lower brightness.
If you want to control the power output accurately then you will have to measure the true rms power by taking many instantaneous current and voltage readings and multiplying them, then sum and average the results over a full mains cycle. Any other method will only be an approximation that could be quite inaccurate at low phase angles.