I worked about 90 hours this month - why was I only paid for 42?

• paid off
• pre paid mastercards
• paycheck loans

I started working part time flexible at Carphone Warehouse. I've been working part time there for over 3 weeks now and roughly I have worked 90 hours already. I have just checked my payslip and I've only been paid for 42.25 hours....significantly lower than expected. Where did all those other hours go? Have I been working for free? I've been clocking in and out of the machine like I'm supposed to so I cant understand why I've been paid for only 42 hours!!!

I worked about 90 hours this month - why was I only paid for 42?