Both the 27++ and 28++ 9-cell batteries are similar, except the 28++ includes hardware authentication which is necessary to work on the new systems such as W530. Either battery will work fine on the W520, which doesn't authenticate the battery. As at July 2012, the 27++ is going end-of-life so it's very possible that customers who order the 27++ will actually get the 28++ along with a flyer in the box explaining this.
In the event where unauthenticated batteries are used, the machine will boot (with a warning message displayed by BIOS and by Power Manager), and it will even discharge the unauthenticated battery. Also, the machine will not charge the unauthenticated battery as a form of safeguard as many of the new systems support RapidCharge, where batteries can be charged up to 80% within 30 minutes.