Because Dr Maynard
The most accurate scientific scales weren’t made for reloading. There is usually a button on them that lets you chose scale Grains / Grams. It’s just the way the better electronic scales come.

40 Rod

Reloading in general has many places to make mistakes. Even seasoned reloaders can make mistakes, but the more we know about various components, the less we're apt to make a mistake. 
I don't shy away from using certain types of powders, but like others have mentioned here, certain powders require more care.

I initially got the digital scale to speed up weighing cast bullets. Since, I've found it to be fine for most of my other uses as well. I used to keep a pair of beam scales at hand, but now only use one, and only as a double check on the electronic one. I used  the electronic one while loading at the range too. It's nice to have unless the wind is blowing. 
If one uses stick powders a good trickler is never a bad addition to the bench, range or home. Unless the wind is blowing.
Lots of things to weigh if you have grandchildren also. Unless the wind is....... Maybe I'm the one causing all the wind! Pat

I agree with Adrian on the cause being double charging.

ever since I have been working with the small cases like the .22 Hornet, I have gotten into the habit of weighing every charge.  I have good accurate measures..Redding and Harrells..but I always measure light & trickle up to desired weight...time consuming?  I guess so, but safety trumps time in my opinion.

40_Rod wrote on May 15^th, 2015 at 9:28am:

Ha, you said it. With one of those scales in GRAMS, i can charge cases without any variation of powder charge. I usually allow about 1/100 of a grain tolerance. My scale has been set to GRAMS for the past 10 years. I don't even know how to switch to something else anymore, but i can do that. I occasionally test it with calibrating weights.  
And because i am a Eurobilly transplanted to the Ozarks. I fit right in with the locals, but i stick to my millimeters and grams. .
There really is no danger of mixing grains and GRAMS. 1 grain is just a few granules of powder, 1 GRAM (15.62 grains) is enough to charge a smaller case and is about a teaspoon of powder. I don't think anyone can charge 15 GRAMS of powder into any sporting cartridge case. That would be enough for an AA gun. 
Really, the metric system is simpler, more intuitive.  
As suggested above, i dump most of the charge with a Lee scoop and i trickle the rest with a trickler, into a small aluminum dish set on the scale. . I seat bullet right after that. Fast and safe, very precise. I usually charge and seat 50 rounds at a time, one by one, in about 20- 25 minutes. In fact, the powders that don't work well in the powder measures, like large flakes and rods, work very well for trickling fast by hand, by tapping the index finger on the handle of the scoop.  
This way, i avoid errors caused by powder measures, i can switch quickly from one caliber/powder to another, and i don't have to re-adjust  a powder measure. Clumsy as i am, i would always miss-adjust the powder measure.  I just have to keep a conversion table and a load/charge table taped to my desk. I also keep the empty, mangled, case that i double charged with Unique one stormy evening. It's right on the scale. I received a severe storm warning on my phone while loading, i double charged. I no longer take my phone to the loading room anymore. 

.22-5-40 wrote on May 16^th, 2015 at 1:21am:

Loading the small cases has taught me a lot about loading the larger ones.
I confirm the charges weighed on an electronic scale with my faithful old Redding beam-balance scale periodically as a double check.
All electronics can glitch. Just think of that e-scale you use as a small version of your home computer. 
I agree that safety trumps time.