With a standard diac/triac setup, the triac won’t turn on until the diac’s forward breakover voltage (V[sub]BO[/sub]) is reached. V[sub]BO[/sub] for an ordinary BR100 diac is 30 V.
On a 120 V system, 30 V is reached at about 10[sup]o[/sup], so no current will flow in the light for the first 10[sup]o[/sup] of each half-cycle.
However, the effect of delaying the turn-on to 10[sup]o[/sup] is to reduce the rms voltage by 0.1 V, which is obviously much smaller than the 1 V reduction caused by the forward voltage drop across the triac.
You need to delay the turn-on to 25[sup]o[/sup] to achieve an effect that matches that of the forward voltage drop of the triac.