Here's a better way to think about it. At best, one of your cylinders can fill 6/8L worth of air/gas. You don't count any certain stroke, just the fact that your cylinder at one point in time can fill only 6/8L of air/gas.
A rotary, although it jumps from chamber to chamber unlike your engine (which uses the same chamber for all strokes) can only fill 1.3L of air/gas. On the next stroke, it's moving 1.3L of that same air/gas, but in a different chamber. But while the air/gas has moved into the 2nd chamber, the 1st and 3rd chambers are not operating. Thus, it would be unfair to count them.
What's important here, is how much air/gas the motor can move in one revolution.
http://auto.howstuffworks.com/rotary-engine4.htm