whats the difference

[Tennis Ball Hitter]

At the gym, I noticed something a bit weird when I am using the cardio machines. I use a heart rate monitor and I notice that when I use the cross trainer my heart rate hovers around the 170/180 mark and I feel like I am breathing quite hard. When I slow down a bit my heart rate goes down and so does my heavy breathing [thisis not the weird part].

But when I am using the versa-climber I notice that my heart rate hovers around the 180-185 (ish) mark, yet my breathing feels a lot less strained and it feels like I could do this forever.

So whats the difference between the two exercisers? why is heart rate not directly related to breathing hard/oxygen intake?

thanks