Haroon,
It's just a bit of maths.
You need to land in 60% of the available distance. That means you must have 1/60x100 of your actual landing distance available. That's the same as 1.66 recurring so 1.67 is used.
For a wet runway we must have 115% of the dry figure so we could do two calculations or we could simply calculate 1.92 X actual landing distance.
For a test pilot actual landing distance = unfactored landing distance. And in many companies this is what is used for line operations.
It could be argued that as we know the average line pilot is not likely to meet the test pilots results we should use more to fully comply with the spirit of the law but you wont find many(any!) operations that include a line pilot factor.