In a HubPages text capsule, a hard return (created by tapping the Enter key) puts in an end-of-paragraph mark. In HubPages, the default paragraph is single-spaced with word-wrap within the paragraph, and double-spaced after.
If a poet types each line of the poem and hits Enter at the end to mark a line break, then it shows up that each line of the poem is a separate paragraph, and it has a full line space after, so the poem looks double-spaced.
If you want single-spaced poetry, there is a solution. Use Shift+Enter at the end of each line of the poem, and HubPages will start a new line for you without the space.
At the end of a verse, use Enter, and you get a space between verses.
If you want to indent a poem, you can try the double-quote button on the formatting bar.
You can see examples of single-spaced poetry in hubs about Haiku by KrisL.