I already know this. Half my family says Istorija, the other half says Historija. I've also noticed that a minority of Bosniaks also use Povijest.
Istorija and povijest, are two different things. Serbs use povijest as well, (the ones knowing when and how to use it though).
Istorija – as in history, is the general sum of knowledge of passed events.
Povijest – is a documentation of a specific event that has passed, as in Povijest of 'something'.
Those two terms are very similar and hard to differ in common understanding, so that the word povijest, with time vanished in the Serbian language (it could also be because of foreign influence upon the language).