Bust a$$ to achieve 70% HR

Not really busting ass, but it seems when I do cardio on the eliptical machine thats actually monitors my HR, I'm working a lot harder as opposed to when I just speed walk w/o a monitor.

I'm only 19, could it be because I have to work a little harder to get my HR up as opposed to some of you old farts (jk), or maybe because I'm in pretty good shape? I'm thinking about doing a slow jog/shuffle instead of speedwalking or mabye alternate back and forth to ensure my heart rate it up. What do you guys think? Thanks bros.