This is a perfect example of how they get people thinking a direct debit offer is great value, but if you sit down and work it out, you start to realise it isn't.
After the initial six months, because they start charging you £16.80 every six issues, that works out at a yearly subscription costing £33.60, so you'd get a better rate by signing up for a whole year at £29.99, which sounds more expensive, but is actually cheaper.
Imagine if maths exams contained questions about stuff you're actually interested in. You'd spend all day with a calculator in your hand, happily working things out.
I know it is actually quite ridiculous...