While its correct that home meters work with in a tolerance of + or - 20% most are more accurate than that and at lower levels were hypos can obviously be dangerous any percentage makes a smaller difference.
ie. Lets assume a meter is out by 6% then a reading at 10 mmol/L could be out by .6 mmol/L + or - whereas a hypo reading of 2.4 mmol/L would only be out by 0.15 mmol/L which would make no difference to your treatment. Even if a meter were out by 10% the difference at a hypo level of say 2.4 would still only be .2 mmol/L making the reading either 2.6 or 2.2 mmol/L.
A meter that was unusually out by the max 20% would make a difference of .5 mmol/L at that same hypo level of 2.4 mmol/L.
The only time it could possibly cause some confusion would be if you meter was out by 20% (most unusual) if your level was say 4.0 then it could be out by + or - .8 which might in these very unlikely circumstances give a false reading of 3.2 meaning that you would treat a hypo that was actually not that low so you would be working on the safe side
If the reading was read as being over then it would register 4.8 which may give you false confidence but if you were about to embark on any physical activity at 4.8 most people would eat some slow acting carbs first anyway.
If your level was actually in hypo territory at 3 and your meter was reading over by 20% it would show 3.6 mmol/L so still showing a hypo level.
Thats how percentages work the lower the number a percentage is applied to the smaller any difference will become.
Hope this makes sense as I am dyslexic and sometimes dont express myself well in writing and ramble a bit.