Apple watch heart rate sensor

I am a little surprised I guess, I don't think it is supposed to work like this. Sometimes i'll just be walking it shows that my heart rate just suddenly jumps up from 70 to 85 then suddenly goes back dow to 70 I don't think it calculated correctly. Or should I wear it tighter? Also I realized even when I am wearing really thick clothing and even a rain coat then wear the apple watch on top it can still detect my heat rate so is it a lie? I then took it off to see if there was even light showing through but there wasn't