How can that be? Heart Rate and perceived effort both fluctuate with a bunch of factors other than power output, and are comparitively poor measures to train by.
This is why people buy power meters.
If a heart rate monitor is all you got, its better than nothing, just don't get to obsessed with the numbers, and realize they are going to fluctuate.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.