He doesn't, or at least not that 80 million.
When you sign a record contract (among other things) the headline figure is both a total which you will only be paid once certain targets are met plus it's considered to be an 'advance' on the cost of any recordings made under the contract i.e. the record company pay for the album to be made but they take the costs out of the artists 'share'. The record company essentially passes most of the costs on to the artist, while retaining a large chunk (usually the majority) of the profits. I remember reading the transcript of a keynote from some while ago in which the accounts of an american band's first album was broken down - the album made $1m profit in total, but the musicians would actually have made more money if they'd been working in MacDonald's at the time. Plus williams didn't write the songs on the albums so he doesn't get writers PRS payments. Given the generally unsuccessful nature of Williams's career since he signed the contract, I'd expect him to have made less than £8m, let alone £80m. the fact he was too stupid to realise the actual situation when he gave the press conference and proclaimed how incredibly wealthy he now was (great piece of pr Robbie, really) just reflects badly upon him.
The fundamental point you have to remember is this : record company executives = vast sums of money for little or no work. Frankly, they're the only people who are stealing a living even more than share traders.
Jon