That is absolutely the case, but the reasons can vary. From improperly tagged artist and album title to differences in track number or duration, inconsistencies in disc numbers…
You have to decide whether you want to invest a fair amount of time into manually trying to identify albums. In my case it was pretty successful as I managed to identify more than 90% of formerly unidentified albums of my core collection (the boxsets and alike I had sent to be exiled).