The reason is a curve in Adobe Illustrator is made of 2 anchor points and 1 or 2 handles, and we simply cannot find a Mathematical formula to measure it accurately. The way we measure it now is to divide the curve into many small segments and measure the length of each segment as a straight line. Therefore the result is just very close to the actual value (99.986% accurate)*. If you compare the results with that in Illustrator's Document Info Panel > Objects, you will notice that our results are basically the same as those calculated by Illustrator. Therefore we think this should be an acceptable solution to measure the length of a curve. Unless your work demands a very high accuracy in the length of a curve, we think that should be ok to use.
*how do we come up with 99.986% accuracy? We take the arc of 1/4, 1/8, 1/16 of a circle and record the results made by Specees, then compare them with the results of calculating them in a simple Mathematics way: 2 x radius x pi, then divided by 4/8/16. The differences are around 0.014% therefore we say that it is 99.986% accurate :).
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