By cumulative, that means the total BTU's that were output over the entire course of the burn of EPA test load. In other words, if they put in 200k BTU's worth of wood, each stove should then have produced something just under 200k BTU's of heat (under 200k because there will obviosuly be some loss of BTU up the chimney). One stove (lets call it stove YY) might burn all the wood in 1 hour, in which case the YY stove put out ~200k in one hour. Another stove (lets call it stove XX) might burn it over 10 hours, in which case stove XX put out the same ~200k, but did it over 10 hours. So in your words, you could say "they both put out the same amount of heat or BTU's". Right? Great. But that is not what the chart is showing. What it is showing is, stove YY peaked at 200k output for one hour (it put out 200k in one hour). Since stove XX put out 200k in 10 hours, you can derive that 200k/10 hours = 20k peak output per hour. Hence, on the graph stove YY would be plotted at 200k BTU's per hour max heat output, while stove XX would be plotted at 20k BTU's max heat output. In the interest of simplifying the explanation, I of course smoothed the heat output lines over time (which we all know is not the real case), and assumed similar heating efficiences between the two stoves (which we also know is not the case). But you get the idea.
So both stoves (XX and YY) put out the same amount of heat with the same load of wood. The difference being, stove XX put it all out at once, and the stove YY spread it out over time. It is only showing one snapshot of the potential of the stoves, which of course happends to be the one the EPA is interested in, a full blast burn (since they want to see the emmisions at this burn rate). Does it tell you all you need to know about a stove? Absolutely not. Does it give one a "reference point" (ability to compare to other tested stoves) of how much heat a stove can produce at max output with EPA test wood? Yes. Is this # the same number you will get in your home? No. But again, it's a reference pioint to be used for comparisons only. From reading the chart, I now know that stove XX can blast more heat in a short period than stove YY if required. That can be a good thing, and that can be a bad thing. It's not saying a stove is better than another.