I think that you are correct that part of the shrinkage of the differential was due to the increased certainty of the deal, but your math is off. You talk about $1 billion differential, but that isn't the case. The bulk of the difference between $5 billion and $6 billion is based upon the share count. Your calculation assumes outstanding shares (78.14mm) instead of the fully-diluted share-count of 89.06mm(which incorporates options).
aQuantive/Microsoft Clears Anti-Trust Hurdle, AQNT Gains $1B [View article]