How to read signal from Crankshaft Postition Sensor in motorcycle
It's called aliasing - if you don't convert the analogue signal to digital at a fast enough rate you get near enough garbage. Consider this picture of a sinewave sampled at too low a rate: -

http://www.iu.edu/~emusic/361/images/digitalaudio-aliasing.png

Imagine the brown waveform is your analogue signal. The arrows are when the Arduino samples the brown signal and the blue dotted line is what you might construe those samples to mean i.e. nothing like the original signal because you are sampling at too slow a rate. You need to "sample" at a rate that is greater than twice the highest frequency of interest such as here: -



However, you should be able to see that the above sampling will miss nuances in the waveform that change too quickly.