There are also differences in the implementation of the achievements - nearly all the new XCOM achievements, like most of the original ones, are things you get through simply completing a playthrough (such as destroying the EXALT base, or using gene-modded soldiers), while Civ's pretty much done all of that and the BNW achievements tend to be highly specific (airlift a civilian unit from Casablanca to Lisbon, as Morocco; excavate an artifact in Egyptian territory as America, with a German unit within two tiles). I have over twice as many hours on Civ V as XCOM, possibly three times as many, and overall have about 45% of Civ V achievements (fewer from BNW than G&K, fewer from G&K than vanilla) and 80-90% of all XCOM achievements. I don't have many of the "easiest" Civ V achievements, typically through not playing that civ or not choosing particular routes (such as Order for a Cultural victory or whatever).
I just checked what the most accomplished G&K achievement was and the most accomplished BNW one.
G&K is Gad Zeus!: Found a Pantheon 33.2%
BNW is Highway Robbery: Plunder a Caravan Trade Route 7.2%
Also, for comparison, the Vanilla achievement Harder, Better, Faster, Stronger: Upgrade a Unit was been achieved by 65%
Upgrading a unit & founding a pantheon are both things that anyone who tries to play even 1/4 of 1 game should get. Although, for the completely clueless, upgrading a unit might be more likely than getting a pantheon.
Plundering a trade route can be done without any particular planning and by any civ in any given game, but it's still very possible to beat the game without doing it. In fact I personally found plundering trade routes a bit difficult until I found and turned on the option to display them on the map. Also, I would only really want to plunder trade routes of enemies.
I think it's fair to say that 50% of people who played vanilla played g&k. I don't think it's reasonable to say that 20% of people who played g&k played bnw. The bnw achievements just aren't automatic enough.
I found something else in the stats which I thought was interesting, but not necessarily related: % of players who have beat the game on each difficulty:
chieftain 22.10%
settler 14.0%
warlord 14.0%
prince 13.50%
king 5.70%
emperor 3.10%
immortal 1.60%
deity 1.30%
Unfortunately, if you beat the game on prince, for example, you don't automatically get the warlord, settler, & chieftain achievements.