Advanced award of PIP on Supersession after voluntary claim/review of DLA. DM decides DLA ends and PIP starts at a future date

Scenario is that the claimant is in receipt of DLA LRM LRC.  Claims PIP in January and is awarded EHRM and EHRC in letter dated 19 April but starting on 20 May   DM decides DLA ends on the 19 May and PIP starts on the 20 May. N.B. There is no anticipated nor forecast change of circumstances. We are in a PIP area since November last year.
My understanding is that a DLA Supersession decision on review of DLA to PIP cannot be backdated for more than 4 weeks. However, what is baffling me, maybe senior moments why the DM decided that DLA ends and PIP award starts one month hence. Should it not be that DLA ended 4 weeks previously on the 19 March and that PIP commenced the next day?
Is it possible the DWP are doing this to prevent claimant’s getting more than 4 weeks back pay rather than making a retrospective award which with delays would mean back pay for a longer period than 4 weeks?

Unfortunately that is the way the legislation is written - see discussion thread here http://www.rightsnet.org.uk/forums/viewthread/6464/

When there is a change of circumstances on DLA and the claimant is assessed for PIP the effective date is 4 weeks after the next payment date after the date of decision. So I’m guessing your client’s payment date would have been 22 April and the change to the new PIP award starts 4 weeks later