Well this is relevant because according to the data, it could be argued that more people that actually started COP(and had the means to finish COP)...finished COP than either of the other two expansions. If we use our little friend math that 27.34% might seem small but if you think of the much larger % of population on COP due to nearly everyone having started the expansion.
Pretty much anyone around let's say level 30 started COP (really most people accidently start much earlier), according to the data that's 44.47% of the population. Now remember COP you can't finish till you're around 70-75 anyway so of that 44.47% of the population that could finish COP only 13.94% of the entire population could feasibly ever finish it. So of course the data is going to be skewed in the "people start COP but people don't finish COP". Only around 31% (and probably slightly lower) of the people that start COP could eventually finish COP simply due to levels.
While on the other hand take ROTZ which as I said is started by about 23% of the population and you can finish it at 70-75ish as well. Well that 13.94% of the population also makes up over half of that level range so once they have the means to start it they are well more likely to end up having the means to finish it, and yet even if you add the numbers to Awakening and The Last Verse, it only comes to about 35.34%. So while over half of the people (About 60.6%) that start ROZ have the means to finish ROZ only 35.34% actually do finish it.
So basically what I'm saying is the number of people that started and had the means to actually finish COP (read as high enough level) is actually quite large in comparison to those that start ROTZ and actually finish it. Because you're looking at nearly double the sample size overall. So if 27.34% of the population from 30-75 (and remember COP could only really be finished in early 70's at earliest) ended up finishing it, that's a pretty huge number. So what you should draw from these numbers is that COP actually was the most finished expansion by people that had the jobs high enough to finish it. Or you could also draw that a huge part of the population simply don't do expansion's storylines.
To sum up in a nice little bow if you don't want to read the novella above:
Around 31%(probably lower) of the people that started COP could realistically finish it, 27.34% did. Of the 60.6% of people that start ROZ and could finish it only 35.34% of people do. Pretty simple math. If you need me to explain it any easier let me know.