Preflight current version is based on ISO 19005 and I understand that version accepts PDF 1.4 and further, but I am not sure of my english, can you validate that point in the specification ?
If 19005-2 allows older versions of PDF, it can be a reason. Reading PDF Reference third edition (defining PDF 1.4), I found DestOutputProfile was something new in that version. That's why they are not present in olders pdf. in ISO 19005, DestOutputProfile is mandatory.
In the xmp block of paed, the value of pdfaid:part is 1 (for part 1 of the specification), so, in my opinion when doc converter 10 changed the pdf to pdf/a it did not change the PDF header and did not change anything in color spaces.