All chains have tight and loose spots.

The more money you spend on frame alignment, bottom bracket, crank, chain ring, cog and hub, the more even your chain tightness.

All the tiny increments add up; all the out-of-roundnesses and misalignments can periodically conincide in one big tight or loose spot; and, they can all cancel each other out.

You can chase it forever and you'll never get it perfect, but you can make it better.

I can't prove it, but I think having prime numbers for both the cog and chain ring can help (47 X 17).