IT sounds pretty normal to me. I've been growing beets for years - one of my favorite things to grow. I think it has to do with the depth the seeds were sown. Usually, the tops protrude from the soil as they get bigger. The ones below ground aren't big enough yet or planted too deeply. The ones completely above ground could have been disturbed when you thinned the crop or just wasn't planted deep enough.

I agree with Bathgate. don't do anything with them just harvest as and when you want to eat them.

Thanks guys, i'll keep my eye on them.