How exactly does the last active time stamp on Facebook work?

[Sanity Not Included]

My girlfriend and I have had heated arguments because of the last active time stamp on Facebook constantly changing from "10m" to "1m" or "14m" to "1m" for hours on end through days, not often do I see an accumulation of more time or hours on the last active time when either one of has to leave and later return. The time stamp changes back to "1m" and personally I'm wondering if she's somehow online without the green dot appearing in the chat bar and ignoring my messages and replying to someone else, I'd really like to know the truth of the matter if anyone knows why this is happening. Thanks