744 maneuvre margin question
During a recent moment of in-flight boredom I checked the performance manual maneuvre margin speeds for our cruising level and compared them with those displayed on the PFD. The speeds shown on the PFD differed significantly (ie more than can be attributed to the thickness of my pencil) and seemed to represent a 1.2 margin rather than 1.3. This was on a RR powered 744. I tried it again on a later trip on a GE powered 744 and the 1.3 figures matched perfectly.
Would anyone be able to shed some light as to why this may be the case. Could it be to do with certification? Is the 1.3 margin a legal requirement or just a convention?
Thanks.