Seeing the number as often as you do probably is related to the curiosity or mystery associated with seeing the number. It can be a circular thing. A question about something can make a person notice the thing about the question more often.

Let me speculate a scenario, not that it happened that way but as an illustration for understanding.

A number is noticed. Many numbers are noticed during the course of a day, but generally they aren't remembered.

But this number, the number being noticed now, is associated in some way with the person's life. In your case, your ex-boyfriend's birth date. For others, it may be the number of siblings they have, their own birth date, a prominent number in their numerology chart, or other association.

The number being noticed and consciously associated with the person seeing the number, tends to be remembered.

The next time the number is seen in the person's environment, it's associated in a similar way as before and also associated with the earlier time the number was seen. It's now remembered much more solidly.

The number is unlikely to be noticed every time it's in a noticeable place. But it is seen now and then.

After a while, it seems like seeing the number so often must have a meaning. And the person really wonders about it. It becomes a big mystery.

The moment the reason for seeing the number becomes a mystery, that's when they start to see the number more often. It's not that the number occurs more often, just that it's occurrences are seen more often.

The mystery perpetuates it. See the Why People See Certain Numbers All the Time article for further explanation why.

In case you're wondering, the meaning of the number 427, among other things, has to do with focus and pragmatism.

The number 427 isn't in your numerology chart. Therefore, I'm fairly certain it's the recognition of 427 as your ex-boyfriend's birth date that attracts your conscious attention to it.