David Mendosa has a blog post about some causes of wacky blood sugar readings. A quote:
Saying that operator error is the biggest problem that people who have diabetes have when we check our blood sugar sounds like blaming the victim. But I’m convinced that some mistakes we make when using our meters and test strips and lancets is the reason why testing so often gives us wacky blood sugar numbers.
Diabetic Mediterranean Diet 
This is interesting article. Once one has followed a proper procedure of cleanliness cleaning fingertips and fresh lancets, there are still issues not described here that are real factors. The article is attempt to use sophistry over failing to provide items that cause trouble.
Having used many of the meters and settling on one that works better – nee more accurately and measures only glucose D and has resistance against interferors, then we can have rational discussion.
Many meters/strips are reading a combo reading of all sugars floating by.
My gut does not play nice and when my system runs into the man made sugars, they get thrown into my blood stream for another pass through the liver and show 40 to 100 points too high 2 hours after a digestion cycle. AT first time in the morning, all meters seem to read accurately until I eat and depending upon the man made sugars ending up in blood stream, then meter wonders higher.
Other factors there have to do with the oxygen/water content with some meters being much more sensitive . Also the hematocritic readings of ones blood also send readings off. As best as I know only very expensive hospital machines have extra well on strip to read blood hematocritic numbers and correct accordingly.
In addition, the blood system can be all over finger to finger, hand to hand, if one has been sitting around. It is most consistent after exercising strongly.
Lastly the blood system contrary to all the blather out there is a highway like ethernet that sees glucose, water, glucose/sugars, oxygen dumped on it and not stoichemetrically ( evenly and consistently) mixed like a evenly shared token ring system and thus readings can bounce around, minute to minute, reading to reading on the same finger and close test spot.
SImple answer is – been there – done that. Simplistic answers and shophistry while well meaning can lead folks astray.
The other comment I would add is that meter/strip technology relying on tiny test dots of blood are extremely sensitive to any trace of containiments left on uncleaned skin. Cleaniness, fresh lancet and fresh dry test strips most critical.