Talkers vary in their vowel pronunciation. One hypothesis holds that listeners achieve robust speech perception through pre-linguistic normalization. In recent work (also submitted to ASA), we modeled listeners’ perception of naturally produced /h-VOWEL-d/ words. The best-performing normalization models accounted for ∼90% of the explainable variance in listeners’ responses. Here, we investigate whether the remaining 10% follow from (1) other mechanisms or whether (2) they reflect listeners’ ability to use more cues than available to models. We constructed a new set of *synthesized* /h-VOWEL-d/ stimuli that varied only in F1 and F2. Unsurprisingly, listeners (N = 24) performed worse on these synthesized stimuli than on the natural stimuli (estimated as inter-listener agreement in categorization). Critically though, we find (1) that the same normalization accounts that best explained listeners’ responses to natural stimuli also perform best explaining responses to synthesized stimuli; (2) the best performing model again accounted for ∼90% of explainable variance. This suggests that the ‘failure’ of normalization accounts to fully explain listeners’ categorization behavior is *not* due to restrictions in the ability to feed our models all available cues. Rather, normalization alone—while critical to perception—seems insufficient to fully explain listeners’ ability to adapt based on recent input.