Yes, unlike Income Tax, NI does not operate cumulatively. If you earn £8,000 in a week but nothing for the rest of the year you will pay no income tax because £8k is below the annual Personal Allowance of £11,500. (Well you might pay tax depending at what point in the year you earn your £8k, but you'd get it back). However on that same £8,000 you will pay around £228 NI. Even though this is below the £8,160 annual allowance before you pay NI it is above the weekly NI free allowance of just £157. So you won't get your £228 back.