Everyone loves practising ásana (physical positions); they are amazing; they make your body strong, supple and definitely healthier. But why does almost every style of yôga practise ásana in a different way?
I can not tell you much about the others methods, as I don’t practise them, but I can explain ours and why is so effective.
India is, historically, one of the most invaded countries in the world. Every great emperor in the world passed through there, including the Aryans, later Alexander the Great and most recently the British invasion. You can be sure that every single colonizer somehow changed and influenced the Indian culture and, consequently, Yoga. The most recent, and significant, change that happened in Yôga was the introduction of repetition of the ásanas. 
Ásana was traditionally practised with permanence, but when the British army landed in India the yogins of that time seemed to like the Western idea of working out the body with repetition and this new habit got stronger when Yôga was exported to United States.
So, what do we lose by practising ásana with repetition? First of all, ásana should not be categorized as “gymnastic”. Ásana has not only the aim to make your body healthier, but, mainly, to achieve body-consciousness and self-knowledge. Body and mind being closely interrelated, the practice of ásana aims at mastery over the body with a view to securing corresponding mastery over the mind. Control of nervous and vital energies produces control of mental functions. The mind-body complex is brought under perfect control, the indwelling Self shines out and the higher self is realized.
I think it is only possible to achieve such delightful results with permanence of ásanas, never with repetitions.
When you stay in the position for a good time it acts deeply on the muscle fuses, the receptors inside the muscle cell that are connected with muscle tone, and with the protection against the risk of strain. The position, combined with focused attention and coordinated breathing, affects the muscle fuses, stimulating them to lower their defensive control over the muscle. The fibres of the muscles decrease their level of contraction and effectively stretch.
DeRose Method Culture 