Bee, according to a telly program I watched (coincidentally) last night, there are quite a few people in the community who believe they are "normal" and thus unable to experience synaesthesia but who show up as experiencing it when tested. The most common test (apparently) for people who believe they don't have the ability, is one where the subject is shown letters of the alphabet (one at a time and randomly displayed) and asked to choose, from a palette, the colour they think best 'suits' that letter. Various tricks are used to prevent memory from playing any significant part but at least 1% of the population routinely and repeatedly associates colours with texts and/or numbers.

Cheers, Rowan