This may be an over simplification but a cam lobe has to fit through the cam bearing in order to facilitate installation. The cam bearing diameter is determined by the design of the block and this restriction limits the bearing size. Therefore in order to obtain the amount of valve lift desired the cam lobe lift has to be increased by the ratio of the rocker arm. More lobe lift can be obtained by using a special grind reduced base circle cam and longer pushrods. This is an extreme case though.
On a 1.5:1 rocker arm the valve end is longer and moves 1 1/2 times the distance that the push rod end moves. For example, if the cam lobe lift is .300 then the valve will open .450. As for why a particular ratio is used it is a balance of geometry and physical design parimeters. The valves need to be set in the head at a specified angle and the lobe can only be so tall so a 1.5:1 rocker will do the job without putting the pushrod, rocker and valve stem at too great of an angle. That's why 1.6 is about as far as you want to go on a small block although I have heard of1.7s in some cases with modified valve covers for clearance.The valve stems would have to be angled away from the rocker studs to use higher ratios and keep the rocker tips centered on the stem. That would require different head castings.
I hope this helps,
Tom
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