It's MARGINAL tax, not total/average tax.
It's perfectly possible to work, say, an extra shift, and be worse off as you lose more than you've earned for that extra shift in terms of tax/nic on the wages, loss of some personal allowance, loss of free childcare, etc.
Just the same as can happen at lower end of the scale when someone on benefits loses more than they earn due to loss of benefits such as housing benefit, free prescriptions, free dental care, reduction of tax credits/universal credit, loss of discounted council tax, etc., plus tax and NIC on their extra "shift".
That kind of thing drives behaviour that's detrimental to UK PLC as it's a real disincentive to earn more/better yourself.
I'm an accountant, I see it virtually every day - people take steps to reduce their income to be better off, whether it's refusing extra shifts, paying a bonus into a pension scheme, or whatever - the end result is less tax/NIC to the treasury.
It's exactly why tax revenue fell when Brown introduced the 50% income tax rate and grew again when the Tories reduced it to 45%!
No one ever said you lose 100% on ALL your wages - it's on the "extra" when you're wages/income is just above one of the stupid thresholds/cliff edges.