Proactive AI writing assistants need to predict when users want drafting help, yet evidence about which contextual cues drive these preferences remains limited. Through a factorial vignette study with 50 participants making 750 pairwise comparisons in professional-email scenarios, we find that compositional effort is the strongest measured predictor (𝜌 = 0.597), whereas urgency has a near-zero scenario-level correlation and no reliable effect across inferential analyses. More critically, we observe a striking stated–revealed gap between abstract factor rankings and situated choices: they rank urgency first in self-reports despite it being the weakest behavioral driver, a contrast at the extremes even though mean stated urgency and effort ranks are close, i.e., 2.06 vs. 2.18. This misalignment has measurable consequences. Within this controlled professional-email testbed, the behavior-derived rule agrees with 61.3% of the 750 observed pairwise choices, compared with 57.7% for the stated-preference rule. This 3.6-point difference corresponds to a net gain of 27 matches. It also identifies a concrete design risk for proactive email assistance: stated rankings can place weight on cues that do not predict situated choices. Future studies should test whether stated rankings create similar design risks in other writing tasks.