Certain numbers being repeated in an environment where many, many numbers are generated is expected.

What's unusual is that you notice them.

My thought is that you notice them because your consciousness is sensitized to them.

Being your birthday number probably isn't sufficient to keep you noticing the number again and again. What can do the trick, though, is to notice it once and have a reaction to it. Like, "Hey, it's my birthday!" Then it happens again, and you start wondering about it.

The more you wonder about it, the more you notice the number when it appears.

It becomes a mystery. The mind doesn't like a mystery, it keeps putting your attention on the number in an attempt to solve it.

It's even gone so far, in your case, of determining the number when it's not the number by using division to reveal it. If you keep going forward, dividing and multiplying by different factors, you'll eventually end up finding a 1 and a 3 in every amount you ring up in the register.

The thing is, it has no special significance. But the mind insists it means something and keeps on trying to solve a mystery with no solution.

Now that you understand what's going on, you have more control. You'll still see the number, but you'll know it has no special significance. It's usual in your job that the number comes up from time to time.

It has to. There are so many numbers generated it's rare when the number doesn't come up. Sometimes it will come up often for a while, sometimes not at all for a period of time. It's the natural way of numbers when they're generated more or less randomly.

Knowing that, you may gradually notice the number less and less. It will still occur just as often as it otherwise would, you'll just notice it less.

Even if you keep on noticing the number every time it occurs, you'll know the only meaning it has is that you noticed something that naturally occurs in the environment of your job.