Nearly half of it is from selling three players who were deemed among our best, the rest is from buying shite and selling shite.
We've got a lot better at getting our money back on players, cf Benteke, El Zhar, etc, but it's not a great trend to be getting into, because the turnover of poor players going all the way back to Houllier is remarkable. People used to defend Rafa because he made his money back on his poor signings, so because we weren't operating at a loss on them, they were deemed good business. It's not really good business when that same money is continuously reinvested with the same haphazard results.
As a side note, I don't really have a problem with our net spend, we've accumulated and invested enough each season to be having some success, I don't think we have to be on a par with United's spending, or City's/Chelsea's, we just need to be better at it, which has always been the case.