Accelerated catch-up charge

Client has catch-up charge of £10,000, ie £1,000 per year. In year 5, he elects to bring into charge the remaining £6,000, to shelter losses. According to the rules, the charge in years 6 to 10 is based on £10,000 less £5,000 (the 'extra' charge in year 5), giving £500 per year, or a further £2,500. But the £6,000 charged in year 5 clears the whole charge! Surely some mistake, or what obvious part of the legislation have I missed?
David Lochhead