Yes they are ultimately predictable, but they are not easily predictable. That is one usually has to go through all the steps of decompression to predict them there are no shortcuts.
Imagine a data stream X that is compressed into another data stream Y. Imagine that a small portion of X is data stream x1 which is the portion of data used for a signature. That will get compressed into y1. Now lets define x2 as all the data in X that is not x1. Now if you are always guaranteed that the same x1 would get compressed into the same y1, then things would be easily predictable and you can just compare compressed signatures. But this is not the case. If x2 is different, then the same x1 can be compressed into a different string.
Imagine a data stream X that is compressed into another data stream Y. Imagine that a small portion of X is data stream x1 which is the portion of data used for a signature. That will get compressed into y1. Now lets define x2 as all the data in X that is not x1. Now if you are always guaranteed that the same x1 would get compressed into the same y1, then things would be easily predictable and you can just compare compressed signatures. But this is not the case. If x2 is different, then the same x1 can be compressed into a different string.