Yeah I've noticed a bit of variance between devices like that. I think it has a lot to do with conduction efficiency. You obviously have your main mod set up how you like it. But your other mods, maybe the connection/firing pin is pushed down a little too far? Or maybe it's a little dirty. Try pulling it up a bit and cleaning it. Or maybe that particular mod just won't make a good connection with the clearo you're using. Not all connection pins are set at the same height, and they can't always be adjusted. Some are set on a spring, and some are not. A clearo/tank/atty that makes a good connection in one mod, may not make a perfect connection in another. Also, different connection pins are made of different materials, that can effect efficiency. Battery power may also plan a role in your mods, although they're supposed to be regulated, so that 10 watts means 10 watts, even if one has a more powerful battery than the other, but *shrug*.
There's a lot of variables. Including how clean your coil is when you use it on your devices. A dirty coil will always under perform. And there's always the possibility that one of your devices is slightly off on the wattage reading. Quality control is pretty good these days, but it's always possible. Especially if you have a knock off, or cheaper brand.