Sorry SJ, I'm not sure why but I had got it into my head that you had been transferred.
My advice stands for DLA as for PIP, in fact your Decision is more difficult with DLA as there is no scale for you to measure how successful your claim was as DLA is not points based.
It is possible that the shorter award has been made to force you into a transfer to PIP in 2017 rather than at a later date, I am not aware of any DWP policy in this area, but that does not mean that there is not and of course it could be just one DM or office working in this way, however you won't get them to admit it either way.
The length of the award is a matter of opinion by the DM based on the medical recommendation and the A-Z of conditions
www.dwp.gov.uk/publications/specialist-g...-medical-conditions/ I would have a look at the entries for your conditions to see what the advice is, it may help you to make a decision on the matter
Gordon