Hitting return is just like clicking the first submit button that appears in the form. This is just the way HTML works. If you want it to act as if you clicked the update cart button, then simply move the update cart button so that it is the first button in the list, currently it probably acts like you clicked the continue shopping button.
hint: you can move the update button so that it is first in the list, but still make it appear in the same position using something like:
<input type="submit" name="WA_Store_Cart_Update_100" id="WA_Store_Cart_Update_100" value="Update Cart" alt="Update" class="primaryCompliment" style="float:right;margin-left:3px;"/>
by adding the float, I can move the update to the front but have it still appear on the page at the end.