Older versions of the CSS specification did not define a pixel to be 96dpi, but most of the existing content depends on this assumption, so using another DPI value would break existing content.
You have 1px = 1/96in (inches) & 96px = 1in due to the definition of the reference pixel.
You have 1pt = 1/72in & 72pt = 1in due to the definition of the point.
Therefore, combining the two equations, you have: 72pt = 96px & (72 / 96)pt = 1px
multiplying both sides of the equation by P (the number of pixels) gives you:
P(72 / 96)pt = Ppx.
You need to make sure that the DPI settings are set to the equivalent settings but the standard DPI is 96.