The "20%" accuracy requirement is only the "region A" of the Clarke Error Grid. In the US at least, meters are (a fraction of the time) allowed to be more than 20% off as long as they don't "lead to inappropriate treatment", e.g. region B on the Clarke Error Grid. So if your bg is really super high, the meter is allowed (at least some fraction of the time) to tell you that it is super high but a number different than 20% from the real value.
As a practical matter both "428" and "302" are (for me at least) too high. Also in my experience... those sorts of numbers come from having something sticky or gooey on my fingers. e.g. both the numbers are wrong and really my bg is likely to be more normal-ish!!!!
See e.g. Wikipedia Page on Clarke Error Grid
This picture shows typical bg meter performance, note that most all the readings outside region A (e.g. more than 20% off) are in region B.