While my teeth generally do not bleed at all when brushing,sometimes(i would say once or twice/month) a gum around one tooth may start to bleed a little spontaneously, without brushing.it is almost always the same tooth(only once it was another).why?

Some localized gingivitis is the apparent cause of the recurrent bleeding. Your dentist can confirm this.

Bleeding means infection. Brushing alone only gets teeth/gums 65% clean. Please see a gum specialist, a Periodontist. And please learn to clean where no toothbrush can reach, between your teeth. Continuing to limit your oral hygiene program to brushing alone will only allow your gum disease to gradually process.