Mathematical coincidence
en.wikipedia.org
en.wikipedia.org
The idea is that there are only so many small numbers, and there are lots of ways of combining things together. The Pigeonhole Principle then says that if you stuff too many things into a small enough space, some of them will be close together. Although apparently obvious, this is more widely applicable than people generally realize. It's used, for example, in one of the proofs that that every prime of the form 4k+1 is expressible as the sum of two squares (Examples: 29=4x7+1=5^2+2^2, 181=4x45+1=10^2+9^2).
Combine this with the birthday problem/paradox[0][1], and you end up with more coincidences than you might expect.
[0] https://news.ycombinator.com/item?id=1312636