So is it also normal for a .3 coil to not register as .3? All the coils I've used register at .27
I'll have to +1 what zoiDman said. It's like that in everyday life too. Things are always "approximated". If it were electronics, then I'd be concerned. But Ecig coils aren't electronics, they're electric which goes by generalized ratings. Kind of like your wall outlet.. it says 110 volts, but it could give anywhere between 105-120 volts. Like your wall adapter, you shouldn't be concerned if it's not exact...but if your wall outlet was giving 80 volts, then I'd be concerned and call an electrician. With cars, If an engine was giving out 248 horsepower rather than the 250 horsepower it was rated for, there'd be no concern... but if it was giving out 200 horsepower, I'd take it to the mechanic.
The ohms has a few things to note however. First, with all coils, the ohms will go up over time as the coil gets used. It'll take a week before you notice ANY change, but it will go up over time if you keep using the same coil. That is why Variable Wattage became popular.. Using Ohms Law, it will vary the volltage depending on the current ohms of the coil to get the same vape.
The other thing to note, is Temperature control works on the Ohms varying in the coils. All coils vary in temperature slightly depending on what the coils are made of. Kanthal is an alloy (a mix) of Iron, Chromium, and aluminum. Because the percentages of the metals may vary slightly from one manufacturer to another, the ohms change ever so slightly depending on which kanthal you get. However, they all hardly change ohms at all and offer a fairly even level of resistance... but it does change slightly depending on heat.. but not enough to make any difference. Temperature control wire uses wire that varies the ohms as it gets hotter ona larger and more consistant rate. Stainless Steel, Nickel, and Titanium are used. Even though Stainless Steel is an alloy (mix of metals), it behaves on a similar rating if you get certain mixes (316L is a specific formula of iron and nickel Chromium, and other substances). NIckel and Titanium is pretty much as pure as they can get and are considered a "base metal"... it behaves on a very predictable manner depending on how warm/hot the wire is. So, as the temperature gets hotter, the ohms go up, and it knows by the difference in ohms how hot the wire is. This is how many digital thermometers work.