Trust your instincts, Julia
-- it's a crock. There isn't any such thing as a 'fat-burning zone' or 'cardio zone'. My personal training textbook had an entire section of the cardio chapter called The Myth Of The Fat Burning Zone
. In a nutshell, yes, you indeed burn a higher percent of calories from fat at lower intensities and a higher percent from carbs at higher intensities ...
You burn the highest percent of calories from fat when you're totally at rest (like in bed) -- a higher percent of almost ZERO calories, that is! Heck of a workout, eh?
The bottom line is - just as you suspected - the total number of calories burned is the number that matters. Who cares about burning 75% of your calories from fat if you're only burning a few calories?? Big deal! Go for high intensity cardio to maximize the TOTAL number of calories burned from ALL sources.
Weight loss boils down to calories in verus calories out and we want to maximize the calories OUT -- that way we don't have to starve ourselves to create a calorie deficit. So forget the zones on the machines and - by the way - you gotta ignore the calorie readouts from the machines too -- they're usually way too high.