If you run a company with wildly varying income then, yes, you may be caught by this. But even then £50k for a few years will happily catch you up. I suspect the numbers caught out by this due to wildly varying incomes are a fair bit smaller than the numbers of people who simply earn large amounts.
One of the reasons for this is to stop directors who massively pad out their pensions when they are appointed - which is something that I've seen happen repeatedly in the finance industry. Look at Fred Goodwin, for instance, who ended up handing half of his back under pressure. Suddenly had a load of cash handed to him, in the form of a ridiculous pension that wasn't based on anything except hiding the income he was getting.
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One of the reasons for this is to stop directors who massively pad out their pensions when they are appointed - which is something that I've seen happen repeatedly in the finance industry. Look at Fred Goodwin, for instance, who ended up handing half of his back under pressure. Suddenly had a load of cash handed to him, in the form of a ridiculous pension that wasn't based on anything except hiding the income he was getting.