My POLAR HRM seems to be pretty accurate also, however sometimes I will get some inaccaurate high readings myself. Very rare occurence, matches my OMRON blood pressure meter. Sometimes static electricity from your jersey can affect readings as previously mentioned.
Not sure if it is me, but I do not wet my chest strap elctrodes on either of my straps(2) & can get a reading at the beginning of my ride or as I am typing this response. If I lay down though, then I can not get a reading.