Paying income tax for one job but not the other

Hi, I’m 18 and I’ve had a part time job since last summer in which I haven’t paid any income tax for. But last week I had a once-off shift for an event company just to earn something extra and on that payslip I’ve paid income tax. I’m a little confused why (sorry if that sounds kind of silly, I don’t really understand finance that much) and if there’s a way to get it back or if that’s how it should be. Thanks.