What may be the reason behind this difference in deviation?

Hint: Spectrum of white is formed by passing white light from a glass prism. This splitting of white colored light into its constituent components is called dispersion of light. It is composed of light having different wavelength i.e. VIBGYOR pattern. Deviation of light is inversely proportional to wavelength of light.

Complete answer:
The white light is composed of different colored lights possessing different wavelengths i.e. VIBGYOR pattern. Red waves possess a comparatively longer wavelength and violet color possess the least wavelength. Violet colored light carries the most energy.
The higher the frequency, the faster the swings and hence the higher the energy.In these colours, red has maximum wavelength and violet has minimum wavelength.
It is a fact that the deviation of a colored light is inversely proportional to the wavelength of the light. Hence, the color with the maximum wavelength is least deviated and the color with the least wavelength is the maximum deviated. Therefore, the deviation of red color is minimum and the deviation of violet color is maximum.
Therefore, this is the reason behind this difference in deviation.

Note:
The difference in the deviations of the colored lights causes the arrangement of the colors in the rainbow fashion VIBGYOR with red at the top and violet at the bottom of the sequence. The difference in the wavelengths and frequencies of the colored lights causes the dispersion of the white light. The formation of rainbow is a natural example of dispersion of white light by the tiny water droplets.