Why were there more than 10 things that she hates in Kat's poem?

As this is based on Shakespeare's The Taming of the Shrew, Kat is likely reciting something akin to a Shakespearean sonnet, which is has this typical form...

Composed of three quatrains and a terminal couplet in iambic pentameter with the rhyme pattern abab cdcd efef gg. Also called Elizabethan sonnet, English sonnet.

Now Kat's poem is a little longer and doesn't have the proper meter or rhyme scheme, but it's not too far off. That being said, there are more than 10 things, because there are more than 10 lines.

I'd like to state that she actually only uses the word hate 10 times, hence the title of the film, but she actually uses it 11...

I hate the way you talk to me

I hate the way you drive my car

I hate it when you stare

I hate your big dumb combat boots

I hate you so much that it makes me sick

I hate the way you're always right

I hate it when you lie

I hate it when you make me laugh

I hate the way you're not around

But mostly I hate the way I don't hate you

Of course, the last time uses it, she negates it

way I don't hate you

So, we'll say that doesn't count, and get to 10, and hence the title of the film.

Of course, in the end the film makers probably just preferred the sound of "10 Things", or were evoking David Letterman's Top 10 Lists, or were anticipating the arrival of click-bait listicles.