Exactly 4:20pm

It means you remember seeing 4:20pm but don't remember seeing the clock time at other moments when you check your phone. And the reason is that you give 4:20pm more mental attention than you give other clock numbers.

It is subjective. Perhaps the number 420 or the time 4:20pm has a special meaning for you, or an emotional significance. Perhaps at a point in the recent past you looked at your phone when it was 4:20pm and suddenly remembered seeing the same time before — and wondered if it meant something.

If that's the case, then know that the more you wonder if it means something, the more you will remember seeing it.

The reason a number is remembered and other numbers are not is that when the remembered number is seen then the person gives the number more attention than they give other numbers. Perhaps it's only "oh, I've seen that number before". Even that is more attention than most numbers receive.

The numbers that get the most attention are the most likely to be remembered.

For more information about how that works, see the Starting to See Certain Numbers Again and Again article for more information about that. And also Why You See Certain Numbers All the Time.