Let's say you get 3 people in a room, and gave them all a quiz question.
Question: How many Litres does this tank hold
It's a hard question. The human brain does not deal with large numbers well at all, and the cylindrical shape makes things more difficult. There is also little sense of scale. Still nonetheless, everyone gives it their best shot.
Person 1 goes first. He has no idea what a litre is, so says 100 litres. Everyone else laughs.
Person 2 goes next. He thinks about it for a long time and then says 1 million litres. Person 1 goes a bit pale in the face.
Person 3 divides the two and says 500,000 litres, confident that Person 1 was over, but not way over.
The quiz master reveals the answer; 100,000 litres.
Everyone stops to think for a second before releasing that Person 1 has won. He's only 99,900 litres out. Person 3 is next closest being 400,000 litres out.
But if you stop to think about it, it's obvious that Person 2 and Person 3 were a lot closer than Person 1. Person 1 had no concept of what a Litre even was.
There are two ways to deal with this; percentages and orders of magnitude.
As a percentage, Person 1 was 99.9% out. Person 3 was 80% out. But Person 1 argues Person 3 wasn't 80% out, but 400% out.
The better way to deal with this is with orders of magnitude. Person 1 was 3 orders of magnitude out, Person 3 less than one order of magnitude out (0.69)
None of this has anything to do with the General Election, other than to say, Diane Abbott's an idiot. I try to defend her, we all have off days, (Barack Obama said he was a Muslim when he had a cold). But there is a non insignificant difference in saying something costs 50000 times less than it will do, and getting a single digit wrong.