Why do red muscle fibres have more mitochondria than white muscle fibre but less ATP than White muscle fibres?
This is because of the difference in mitochondrial protein expression of red and white muscle fibres. There is difference in posttranslational modification, which leads to functional difference as red muscle contraction is slow, and white muscle contraction is rapid. 

Metabolic control in mitochondria contributes in speed of activity and cellular energy production. So, these differences effect the cellular energy levels in both white and red muscle fibres.

Following factors also contribute to the ATP production:
White muscle mitochondria have higher proton leak. 
Red muscle contain high palmitic acid and oleic acid, but white muscle possess more phosphate ions.
Collagen and elastic fibers are largely distributed in red muscle mitochondria, whereas high titin levels and densely packed sarcomeres are present in white muscles. Whereas high levels of long sarcomeres are present in red muscles.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3118618/

https://www.hindawi.com/journals/bmri/2018/5816875/