The question is legitimate but I am afraid there is no convincing answer to it. Comparative Indo-European linguistic analysis is based mainly on regular correspondences among forms with similar meanings. The concept of semantic similarity is not well-defined however. Formal regular correspondences are so many and so clearly not based on chance that the difference in meaning are often disregarded as having little probatory power. In other words, if we observe words in two, or more, IE languages that patently present regular correspondences in their phonological shape, then the meanings can be also quite different, but we will consider them cognate nevertheless.
The situation is especially confusing with pronominal stems: sometimes they change their reference or grammatical characteristics quite abruptly. Usually, all the meanings that are more or less related to pronouns and pronoun-like adverbs are accepted as "corresponding" meanings for some forms under consideration. In other words: reflexive 3rd person pronouns are "interchangeable" with anaphoric/demonstrative 3rd p. pronouns, which, on their turn, are "interchangeable" with adverbial meanings.