Note the word 'analogy'. What do you mean by an audio cable is 99.9999%, this statement in itself makes no sense. Percentage of what? Purity? For all intents and purposes a functioning audio cable is 100% and that is that, even if you were a bat. And regarding your statement about HDMI, that in fact is not true:
It is commonly stated that all HDMI cables are created equal because it is a digital signal. While this is close to the truth when the spec is followed, it's not always true. The one major thing that can really destroy your signal quality is the length of the HDMI cable. It's true that HDMI signals are digital, and digital signals are 1's and 0's. The problem is that there is no such thing as a 1 or a 0 in digital electronics. It is represented in various ways. Lack/Presence of a signal, a positively magnetically charged or negatively magnetically charged medium, voltage at a certain value, etc... For instance a hard drive stores data using magnetism. The signal stored is read against an expected value range. For instance a 1 can be stored at a signal strength of 10 (while a zero is -10). A signal strength of 9.6 will also be read as 1. This is how overwritten data can be recovered. While a hard drive will read something as a definite 1 or 0. The signal strength can be used (with the help of sensitive equipment) to approximate what the previously written value was.
Here is a chart from wikipedia detailing the phenomenon:
Analog signal: +11.1 -8.9 +9.1 -11.1 +10.9 -9.1 Ideal Digital signal: +10.0 -10.0 +10.0 -10.0 +10.0 -10.0 Difference: +1.1 +1.1 -0.9 -1.1 +0.9 +0.9 Previous signal: +11 +11 -9 -11 +9 +9
How does this relate to your HDMI cable? As the length of the cable increases, not only does the signal strength decrease but so does the differentiation between each subsequent bit. If the signal quality is so bad that the machine at the other end cannot tell where one bit starts and another ends, it can guess (based on the signal strength) an incorrect value. The resulting signal is still digital, is it not? And yet it is incorrect. A poorly constructed cable’s signal degradation is affected by this problem, while high-quality cables often have active boosters. Thanks to this the signal strength stays at values that can be properly read (as a 1 and a 0) and will not blend with their neighboring bits.