Fun fact: The “Uncle John” who compiles these readers is acclaimed sci-fi writer John Scalzi

Wait, so then what’s the difference between a bar and measure?!

I think I get it, so a whole note is 8 beats and there’s 6 to a measure, so the whole note carries on for two beats into the next measure, ya?

Yeah a lot of ways to write it, and that’s part of it - depending on the tempo and phrasing it may be easier or harder to read the music in different time signatures, so there’s some flexibility. Like in 3/4 if you have phrasing that would carry a note from beat three across to beat one of the next bar, in a lot of places, that’s messier to write and read than if it’s e.g a quarter note within a 6/8 bar.

And a whole note is always the length of the bar, so in 6/8 a whole note isn’t eight beats, it’s six. The bottom number represents what length note counts as one beat - in 6/8 an 8th note is one beat (and there would be 6 8th notes per bar) a quarter note is two beats, a half note is three and a whole note is six.

I’m betting different composers had preferences as well.