In my (albeit limited) experience, it's all been down to the accuracy of the customs declaration on the item being imported.
Anything in the hundreds that I've imported, that has had an accurate customs declaration on it, has had VAT added (I calculated it beforehand, though, using an online calculator, so factored it into the cost).
Other things have varied - especially going back a few years. In the last couple of years, though, the small number of things I've imported, have had the correct level of charges imposed on them - nothing has slipped through - but then the customs declarations were correct.