Why This Happens
To understand the gap, it helps to know how pension tax relief actually works. There are two main routes.
Net Pay Arrangements
Your pension contribution comes out of your salary before income tax is calculated. Relief happens automatically, at whatever rate you pay tax. Nothing more to do.
Relief at Source Schemes
Your contribution comes out of your take-home pay, after tax.
Your pension provider then claims basic-rate relief (20%) from HMRC and adds it to your pot, so paying in £800 typically becomes a £1,000 contribution.
Here's where it gets costly for higher earners. If you pay tax at 40%, you're entitled to a further 20% relief on top, but that extra amount isn't added automatically.
You usually have to claim it yourself through Self Assessment or directly from HMRC. Miss that step, and the money stays with the taxman rather than landing in your retirement fund
Charlene Young, head of technical at AJ Bell, put it plainly: most people assume their pension provider or employer is handling all of this.
For many, that's true. But it isn't universal, and AJ Bell has flagged that Nest, the UK's biggest workplace pension provider with 13 million members, uses a relief-at-source structure.