Heart rate monitors also come “coded” and some not, which means the cheaper models will cross read if you sit close to another person with a heart rate monitor on. This often occurs in group indoor cycling sessions, about one and half metres (5ft,) apart and your okay.
Some times if you wear the receiver on your wrist it will drop out of range and appears if it is playing up, so it is better to stick in on the handle bars.
The chest transmitter needs a little moisture on the surface against the skin to pick up your heart beat.
Maximum and minimum heart rates are very individual specific, I train with a thirty year old female who can maintain a HR of 204 bpm for twenty minutes and another of the same age who goes anaerobic at a HR of 170 bpm.
your max HR will also be different for different physical activities, running may produce higher HRs than cycling for example
Your max HR also goes down (or up ) as your fitness improves.
You can test yourself to establish your Max HR
some more in depth stuff here
http://www.heartzones.co.uk/220_no_merit.pdf