^^If I had to guess, they are able to get more square footage for office space with this redesign. It looks like a lot less mechanical volume than the last design, and given the demand for office space in Boston right now they're possibly aiming to squeeze what they can from this site.
I'll never forget what the development team at Boston Properties said to me at a BCDC meeting about this site: "If height wasn't an issue, we would build a Salesforce Tower [San Francisco] at this location in a heartbeat."