If you look at the value of ARM's $-denominated ADRs

  http://www.google.co.uk/finance?cid=662445

This is basically a measure ARM's value in $. You will see any value in the fall the pound was more than offset by a rise in ARM's share price at the time. As ARM's revenue is largely $ denominated and it has significant proportion of its costs in UK people (£) then the fall in the pound led to a bigger rise in its share price as the markets expected bigger profits.