Quantization-Induced Decision Changes Have Regime-Dependent Intervention Spectra

Quantization can preserve aggregate task performance while replacing individual predictions. However, neither score changes nor disagreement rates reveal how the associated behavioral effects are organized across representation directions. To address this gap, we introduce intervention-spectrum analysis, which tracks paired-decision transfer along ordered shared directions and normalizes aggregate prefix transfer by the complete task-supported transfer rate. Using directions fitted to other folds' full-precision states, we project each held-out paired-state difference onto successive prefixes. On disagreeing inputs, we add these projections at their native magnitude to the quantized residual state, testing agreement with the original full-precision prediction regardless of correctness. Across five checkpoints and three quantizers, final-site task-supported interventions outperform dimension- and norm-matched ambient controls in fixed-choice answer selection and teacher-forced full-vocabulary prediction. Despite this shared directional advantage, quantizer-level summaries show that the first 16 directions attain 91.8-92.1% of the complete task-supported one-way transfer rate in fixed choice, versus 12.6-15.1% in full vocabulary. This contrast persists under plain pooled-PCA, while candidate restriction within the full-vocabulary computation does not reproduce compact accumulation. These findings distinguish directional advantage from concentration: effective transfer can remain broadly distributed along a shared representation ordering. An anonymous code repository is available at https://anonymous.4open.science/r/quantization-behavioral-drift-CD71/.