When people keep seeing certain numbers all the time, they also see other numbers. But they just forget they saw the other numbers.

Same with clock times. When a person keeps seeing certain clock times, they remember seeing those times but don't remember seeing the other times that they also saw.

Intellectually, a person knows they saw other numbers. However, because the other numbers are not remembered, emotionally it feels like the remembered numbers are all that they saw.

So why does a person remember seeing certain numbers? Because, to the person, the numbers are somehow different than normal.

For you, whole clock numbers are different enough for you to mentally make a note of the fact that you saw it. Giving it more attention like that makes it likely that you will remember seeing it. Which is why you keep seeing those numbers.