Although it may seem somewhat flippant, this is the most likely answer: You keep seeing your birthday numbers because they are your birthday numbers.

Here's how it generally works:

You see a lot of numbers during the course of a day. Lots of numbers.

Most of those numbers you forget about seeing soon after seeing them, perhaps immediately.

But some numbers you remember when you see them.

The numbers you remember seeing are numbers that are notable in some way. Examples are numbers with:

• Personal meaning. Birthday numbers are an example.
• Emotional significance, like a marriage date for example.
• Unique construction that makes them remarkable, such as numbers with repeated digits.

Different people remember different numbers. Some don't remember any at all.

When a person remembers a number, they remember seeing it.

When a person sees a number (remembers seeing it) but doesn't remember seeing other numbers, it seems as that number is the only one being seen. But it only seems that way; it's not the actuality.

Now, when a person keeps seeing certain numbers, such as birthday numbers, and then they begin to wonder why they keep seeing it, they give the numbers an emotional significance. With both a personal meaning and an emotional significance, it's much more likely those particular numbers will be remembered every time they are seen.

The articles Why You Start Seeing Certain Numbers Again and Again and Why People See Certain Numbers All the Time have more information about how that works.