Knee extension ROM prone vs supine

Hi guys. I wanted to get the thoughts of different therapist on this issue that, surely, everyone has encountered. It relates to knee extension ROM in post surgical patients performed in supine vs prone. Why is supine so much more painful and limited than prone? I can take a TKR/ACL into full extension in prone without pain, flip them over and suddenly they're short by 7 degrees with pain! At first I thought it was just measurement error but I prop the knee, avoid hip compensation and align properly. Then I thought maybe it's hip flexor tightness but that doesn't make sense because the hip is in extension while prone. Hamstring tightness wouldn't be the source. It's more of a curiosity than a problem. They other clinicians don't have a satisfactory answer. What do you guys think? Have y'all noticed this?

Brett

Quadriceps is a two-joint muscles. In supine hip gets flexed too while flexing the knee. In prone the hip gets locked in the extended position. The only movement of the quadriceps happens at the knee joint. Hence the range reduces.

Thank you for your response, eezalign. Perhaps my description wasn’t very clear. I’m referring to obtaining full knee extension in both positions. So the only real difference is being prone or supine. The hip is in neutral both ways as the knee is stretched into extension.