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I've been reading docs Best practices for writing Dockerfiles. I encountered small incorrectness (IMHO) for which meaning was clear after reading further:

Using apt-get update alone in a RUN statement causes caching issues and subsequent apt-get install instructions fail.

Why fail I wondered. Later came explanation of what they meant by "fail":

Because the apt-get update is not run, your build can potentially get an outdated version of the curl and nginx packages.

However, for the following I still cannot understand what they mean by "If not, the cache is invalidated.":

Starting with a parent image that is already in the cache, the next instruction is compared against all child images derived from that base image to see if one of them was built using the exact same instruction. If not, the cache is invalidated.

That part is mentioned in some answers on SO e.g. How does Docker know when to use the cache during a build and when not? and as a whole the concept of cache invalidation is clear to me, I've read below:

When does Docker image cache invalidation occur? Which algorithm Docker uses for invalidate cache?

But what is meaning of "if not"? At first I was sure the phrase meant if no such image is found. That would be overkill - to invalidate cache which maybe useful later for other builds. And indeed it is not invalidated if no image is found when I've tried below: