How was a higher price possible?

Many people were under the impression that Staples would not be in a position anymore to increase the offer price further, after they increased the price from EUR 8.00 to EUR 9.15 a week ago. The Dutch Decree on Public Takeover Bids (Besluit openbare biedingen Wft) gives a detailed overview of the process, and both parties have referred to it in their communications with the public. Article 15 of this decree states that the offer price can be increased once, and Staples used that opportunity already. In the news release from CE we can read that article 19 offered another opportunity to increase the price:

Prior to finalisation of the agreement with Corporate Express, Staples Acquisition BV purchased 2,085,403 Ordinary Shares, representing approximately 1.1 percent of Corporate Express’ outstanding ordinary share capital, for a price of € 9.25 per Ordinary Share in a transaction other than a regular on market transaction. Therefore, in accordance with Article 19 of the Dutch Decree on Public Takeover Bids (Besluit openbare biedingen Wft), if the Offer is declared unconditional, Staples Acquisition BV will be required to pay (i) € 9.25 per Ordinary Share validly tendered into the Offer (or defectively tendered, provided such defect is waived by Staples Acquisition BV) and (ii) € 9.25 per ADS validly tendered into the Offer (or defectively tendered, provided such defect is waived by Staples Acquisition BV).

The number of shares purchased by Staples in this transaction comes very close to the amount of share options, issued by CE, that are in-the-money. This could of course be a coincidence, and assuming that Staples agreed to buy the exercised employee options would only be speculation.