Double-spaced posts

Why, when I type a single carriage return, does the board insert two?
This sentence should be directly below the one above it.
Instead there will be a double-space.
Why?

I was thinking as I typed the OP, “I just know this is going to show up single-spaced.”
And son of a b*tch.
It’s single-spaced.
And MPSIMS, which is where I see it happening most frequently, apparently just went down so I can’t link to a thread which exhibits it. If you care to go to the “Funny Sequential Thread Titles” thread and look at my posts there, you’ll see what I mean.

You mean like this?