I think that you are right, its HMRC applying the triple lock increases to every recipient of the SP irrespective of whether they are actually in receipt of the increase or not. When in touch by telephone they will always refer you to the DWP to have the issue corrected but the DWP will only send you a statement to show what your SP is for the current tax year unless you also request previous years.
You then have to forward to HMRC, so in effect you are doing the DWP's work for them UNLESS you make a complaint direct to HMRC, that seems to work (see my and BM MartinL 's HMRC reply)