The market was expecting a higher bid, so AOE was selling at a...

The market was expecting a higher bid, so AOE was selling at a premium to the Shell offer at up to $5.30. Now, the market believes that the best bid is in and AOE is selling at a discount to the offer (if you take into account that Dart would be worth a lot more than 30c), perhaps reflecting a risk that the takeover does not proceed for some reason.

Note that Shell is in a unique position to offer extra value in Dart by removing the options Shell has over AOE's international projects outside India. Another bidder could not offer the same deal and would therefore have to pay extra in cash (or perhaps shares).