Very odd interview/hiring timeline for physical science field
All I can say is that different universities can take different amounts of time to process things. The process can take even longer when the process drags out over the summer, as many of the key people who need to approve such a hire (and usually there are several) are often out on vacation, which can drag the process out even further.

I know, for instance, that there was nearly a five-month gap between the time I had my interview and was told I'd be getting an offer and actually receiving it. So delays are possible.

However, it may also be possible that you were not the top-ranked candidate after the search, and they are currently waiting to hear back from that candidate before deciding whether or not to make you an offer.