It does change the effective length
The effective top tube length is different it because the top tube is shorter.
17″ AM not being on their website is probably just a mistake on Orange’s part, we’re showing it as an option on our website, and the frames are all the same regardless of spec.
Looks different to me, only a few mm though so you wouldn’t notice, so its pretty pointless.
Frame Size AM 14″AM 16″AM 18″AM 20″
C. Top Tube 442 559 577 601
D. Effective TT 564 580 600 620
Frame Size 14″ 16″ 17″ 18″ 20″ 22″
C. Top Tube 542 557 570 576 596 616
D. Effective TT 564 582 602 602 620 640