Language users systematically prefer one lexical item (because) over another (even highly similar) one (since) to express a causal relationship in discourse. Such choices provide a window on speakers' cognitive categorizations, and have been modeled in previous work in terms of subjectivity. This paper analyzes the Dutch connectives omdat (‘because’) and want (‘since/for’) in written text, conversation, and chat interactions. These can be considered a case in point for linguistic categorization since related European languages show similar distinctions. We sketch a profile for the interpretation of omdat and want based on corpus analyses of large numbers of occurrences in different media and genres. However, we focus on the deviations from the prototypical use of the connectives. We analyze instances of those deviations, in order to be able to understand the deviating use. We conclude that deviations should be interpreted in terms of core elements of the prototypical use. Therefore, the semantic-pragmatic profile of want and omdat should not be considered as hard-wired all-or-nothing rules, but rather as a prototype structure with a core meaning/use and more peripheral uses. The non-prototypical, peripheral uses are motivated deviations: we need the elements in the core profile to understand the deviations.