HIIT Programs benefit Metabolism in a suprising way

High Intensity Interval Training has been shown to increase metabolism up to 48 hours afterwards.

I personally don't feel trainers are any better at weight loss or fitness than nutritionist or doctors. The advantage may well come from bad sampling results.
The people who come to trainers have already overcome the major hurdles in recognition, dedication and accepting help. Hence they experience better results.
Also the fitness trainers likely look the part. It's easier to motivate when people can see their goal. Unless someone in on the ground with you it's harder to be consistent. But good students make teachers look good deserved or not.