Illogical PIP award dates

When I was first awarded PIP my award ran from the date my claim was submitted from March 2015 to July 2019. It said "both dates included" and I assumed that if my condition didn't change in that time period that I'd continue to be awarded PIP, no matter the outcome of any subsequent award review. In July 2018 my award was being reviewed as expected and I was required to attend an assessment in January 2019 where I was awarded 0 points and my PIP was stopped shortly after. I requested a MR and I've just received their decision today, my PIP has been reinstated with backdated payments from January 2019.

Okay, so my new award (or same) doesn't begin from July 2019, as you may reasonably expect given that nothing had changed (I was awarded the same rates) and my original award was until July 2019. So, why doesn't the award apply from only after the current award expires? If my condition had changed in the intervening period and it could be proven that I was only entitled to a lower rate (or higher) PRIOR to the current award expiring then I could accept the "new" award to apply from BEFORE the current award expired.

But get this...I've been awarded PIP from April 2018 to January 2022. So, not only is my "new" award not from when my current award was due to expire (July 2019) it's 15 months earlier!!! That's 15 months of PIP I've been robbed of. Is this lawful? Can the DWP choose whatever dates they want the award to run from in addition to when the award will end (even though it's never honoured!).

What does the law state about award dates though? Even if the "new" award supersedes the old, how is it possible that the "new" award applies from the past? In my case the decision has been made with backdated payments applying from January 2019, how then can the award start date be April 2018? It makes no logical sense.

No, I haven't contacted the DWP about this, is it worth doing? Can the DWP choose the length of the award and the dates that apply however it sees fit? Or are there standard durations that are applied? The reason I ask is because the award duration and dates seem arbitrary. When I was first awarded PIP the award ran from March 2015 to July 2019, that's 4 years and 4 months of award (which became 3 years). Now I've been awarded the same rates, but I've been awarded it from April 2018 to January 2022, that's 3 years and 9 months (but April 2018 to January 2019 (9 months) has past, so it's effectively 3 years (to become 2 years?)).

Is there grounds for appealing the award start date? The only possible reason for it applying from April 2018 is that is when my case could have started to be reviewed, even though I they said they'd contact me only after July 2018 in my original award. Even so, their decision was not until January 2019 so I cannot see this as a good enough reason.