Without a doubt rotor phasing. Another possibility is the other system the can alters which is pickup gap tho usually that change ain't enough to effect a misfire. It's handy to have a hand held vac pump available for dist work as you can see (& measure) the pickup gap change with more vac applied to the can on your bench. To check phasing cut a 1/2" hole in the top flat of a dist cap 2/3 of the way between the center cap terminal and the #1 cap terminal then shine a timing light straight down in the hole while idling & it will freeze the rotor like a strobe light will do to bodies on a dance floor & see how far around the circumference the tip clocking is from dead center on the cap #1 terminal. You can also make a mark on the top metal perimeter of the dist rim at the center of a cap bulge then take off the cap & see how far the rotor is from pointing dead on to it (rotor retracted & not into the springs & they likely will be retracted if free) & the can will shift rotor phasing CCW from that position on a SB and CW on a Big block. You have that circumferential distance and the radial distance from a too short of rotor blade. If you like math you can get the degrees of the can from the number on its arm and the diameter of the dist housing & C=Pi x dia & divide 360 into the circumference to figure the distance around the arc 1 degree is then multiply that number times the # of degrees the can will shift the rotor around the arc. EDIT the X factor is at what distance does the required voltage exceed the available voltage (that's when it startts to miss) & each ign system is different & required voltage is the greatest at WOT & that's when you want the pickup gap to be close to .008 and rotor phasing to be in the good range