OK I can see that happening in the face of a sudden dramatic drop in network hash power, which at this point I find unlikely. Taking your example further, we could say that for a diff of 1.7m, a drop to 3Th/s would need only 3 more Th/s to 51%, however it would be extremely boring because the network would be functioning at half speed and everything would take twice as long to occur.
What I am saying is true even if the hash rate suddenly increases by 2x. If it does, then you would need 24 Thash/s to perform a majority attack, while the difficulty would still be 1.7M.
There. I gave you 3 scenarios where you would need either 6 Thash/s, or 12 Thash/s, or 24 Thash/s, to perform an attack, and in all 3 cases the difficulty would still be 1.7M for up to 2016 blocks.
The bottom line is that the difficulty factor is an average of the last full chunk of 2016 blocks, whereas a majority attack only needs to be executed for a handful of blocks (depending on how many confirmations are required by the recipient you are planning to double spend), and the global hash rate can vary either up or down quickly between these 2 intervals.
