Oh yeah? I challenge your statement by providing a proof of how this digital technology can be off by couple of pounds... if control environment and method of measuring are identical.
On Berkley's website they state the accuracy of this scale is +/- 1/4 oz.
http://www.berkley-fishing.com/prod.php?k=79557&u=CFS50MT
The reason your $5 food scale is a POS obviously 1) it has very inferior components and 2) it uses gravitation force and 3) probably incorporates poor mechanical parts that can influence resistance measurements. Let me elaborate.
1) Cheap digital circuit plus poor electronic gauge basically makes measuring defective.
2) Gravitation measuring requires solid platform and perfect perpendicular stance. For example, measuring your weight (on bathroom scale) can result in different reading between solid concrete and carpet. Go ahead... try this yourself... instant weight loss. Secondly, if the scale is sitting at an angle of 1/2 a degree or more, this can also affect true weight reading.
3) Whatever the device, it still needs mechanical parts to make it work. Ie, flat dish, connection to resistance gauge... etc.
Having said that, a digital hanging scale is not like a food scale sort of. Hanging scale measures resistance from opposite directions, regardless of gravitation force. Therefore, it eliminates #2 variable described above. The main reason the fishing scale is rated to 50lb is because the combination of the handle and metal link hook are made to tolerant or withstand the weight up to 50-ish pounds. More than that it will snap the mechanical parts. Ergo, the 75lb scale uses heavier duty hook and handle.
Up to this point, I still haven't heard your theory how an old fashion sliding spring can be more accurate than a digital strain gauge. Furthermore, it requires one's eyeballing (guessing) it's on mark. Oh, and the only advantage your mechanical tire gauge has over mine is the fact it does not need a battery.
Embrace technology everyone. It's the same principle that brought you Wii FIT.