I think it needs a lot more research on this family There is obviously some confusion as to whether her husband is an Arthur or a John. It looks at the very least as if two people have been conflated.
''John Arthur'' Percy has a death date of 1537 but 'his' will was written in 1508 and probated in 1520 This will is for a John Harris of Prittlewell ( The National ArchivesPROB 11/19/419 Description: Will of Johannes Harreis of Prittlewell, Essex Date: 06 July 1520 . It is viewable on ancestry but it is in Latin so maybe trust the translation/transcription as on the profile)
If William Harris MP b. 1505 is his son which is possible from the 1508 date of the will, then his mother Joan can't have been born in 1525. ( also her putative father, Thomas Percy, was born in 1504 )
History of Parliament actually gives William's birth as before 1502 and names the father as John Harris and the mother as simply Joan. http://www.historyofparliamentonline.org/volume/1509-1558/member/harris-william-ii-1502-56
There is a very short discussion on this family on soc genealogy medieval.
I don't know why but the writer of the History of Parliament entry for William notes in the references "Vis. Essex (Harl. Soc. xiii), 59-60 appears to be a fabricated pedigree, comparable to that of the Westons "
The visitation of Essex page is here