People that own one type of share of Pivotal are getting $15 per share for them. Dell, which owns a different type of share, is exchanging the Pivotal shares for VMWare shares. Because the shares of Pivotal and VMWare have different values, it isn't a 1:1 exchange, the VMWare shares are treated as being ~20 times more valuable.
I would say the transaction is mostly boring accounting, not anything weird. Who knows if Pivotal should operate as part of VMWare instead of independently, but once you decide to do that, you have to consolidate the ownership somehow.
An aside, the difference in the shares is so that Dell could keep control of the company while selling some of it on the stock market. I wonder if there is a good analysis of how companies with closely held control do over time.