Autonomy and tender offers

The FT Alphaville notes an important difference between US and UK tender offers. They point to H-P's announcement that it is extending its tender offer after 42% of shares were tendered. My reaction, like Alphaville's was "that's awful low."  But it turns out, that's good for the UK where tender offers in their first round typically get only 10% of shares.  Now, that's low.  From Alphaville:

A typical level of acceptances at the first close of a UK takeover offer is somewhere around 10 per cent. That’s because investors are reluctant to give up the optionality of backing another bid emerging. In fact, hedge funds almost never give up this optionality until they absolutely have to.

But in this instance almost 42 per cent of Autonomy shareholders have signed up at the first opportunity even though there’s the possibility (admittedly slim) of a counter bid

 Interesting. I wonder what drives this behavior.  The Takeover Panel rules?

-bjmq