Robert Scoble gives some reasons why a company might accept a lower valued offer after putting itself up for sale.
Besides the reasons that Robert gives, there’s also the matter of terms. A cash sale is different than one constructed from stock. This might break either way. A seller might opt for less cash (over stock, let’s say) or possibly “less” stock in the expectation that it has better growth potential than they might be able to achieve otherwise. Generally, the seller is going to pick the “best” deal for themselves–whatever the reaons. The total dollar amount reported in the press can miss the mark in terms of what makes the deal the “best.”