you can easily confirm this by taking your own pulse for 15 seconds and then multiplying it by 4.
I'm not sure what you are trying to worry aboutl
A HR of 95 in a 16 year old is "miles away" form worrying about a too high HR, unless you have very low exercise tolerance or have been told by your doctor to keep you HR below 100, say... and unlikely situation I think.
So what if the tread mill is off by 10%, you could have a HR as high as 105, no problem, even a 20% error, so what?
Now if you see a HR of 200, slow down and if that is when walking you need to work on starting slow and working your way up over a period of months, that is getting into good physical condition.