Not seen that web site before, Trek. Just entered a recent 10K and the calculation it comes up with for me across all other distances and training zones is spot on to what I normally train at and the race times I achieve.
On a separate note - I ran my usual run at usual pace etc today after a week off with the 'lurgy' and right from the start my BPM was up around 11 bpm. This technically showed I was running in a zone close to threshold! Maybe I haven't recovered enough or it was very humid. Fortunately, I know from doing this run a lot that this reading today is unusual. Had this been my first experience with heart rate I too would be baffled