Why do I now need over 6,000 when last week was 3,750?

Why do the amounts keep changing? When I first had the app, I'd had it a week & it went from 3,150 or something to 3,750 to get a £5 voucher. I've noticed today that I now need over 6,000 for a £5 voucher! And was getting excited as I was nearing my 3,750!

My sister needs 5,000 for a voucher. Why are we all different?