Like panache45 said, there is more than one type of synesthesia. I experience lexical-gustatory synesthesia, meaning that some words, whether I hear them, think them or see them on a page, have a taste. The taste isn’t the same as if what it tastes like fills my mouth (for example, I think angry tastes like sausage, but I don’t feel as though I’ve got sausage in my mouth - the sound just evokes the same sensation on my tongue), but it does have a distinct taste.
Interestingly enough, I also have a seizure disorder, which is more common in people with synesthesia than in the general population (or maybe synesthesia is more common in people with seizure disorders?). Anyway, both synesthesia and epilepsy have a lot to do with the wiring of the brain, so I guess it makes sense that epileptics are more like to have it. I’ll bet if more research were done, scientists would find that people with migraines are also more likely to have it.
I’ve had synesthesia for as long as I can remember - even before the seizure disorder developed when I was in my early 20s. Perhaps that was a precursor? Sometimes I wonder if having synesthesia has made me better at languages - I can make some odd word associations. But who knows?
Either way, given that people are so very different and there really doesn’t seem to be any totally normal person without something about them that’s different (depression, bipolar disorder, epilepsy, a skin condition, severe PMS, anger issues), the presence and diversity of synesthesia isn’t all that surprising.