Below is a paragraph in my English textbook:
Andrea - There are six of us in our family, my parents, me, the twins and my older brother, Ted. Ted is a good ten years older than me and the twins – they are girls – are two years younger than me. We had a good childhood out in the countryside. My parents were always busy with the farm and so we didn't get the help with school work and things like that that children get today. But my parents were very loving, especially my father. I regret not helping more around the farm because now I realise that it must have been very hard for my parents, especially as my grandparents lived so far away.
The Oxford Dictionary says the word 'good' in this case means
5.1 Used to emphasize that a number is at least as great as one claims.
‘they're a good twenty years younger’
The Longman Dictionary also gives the similar meaning:
a good three miles/ten years etc
at least three miles, ten years etc, and probably more
He’s a good ten years younger than her.
However, I feel it quite strange. Because if Ted is a person who Andrea hasn't known yet, then she can guess he is at least ten years older than her (by looking at his appearance or his photograph). In the passage above, Ted is her older brother, then Andrea must know exactly how old Ted is. He must be ten years older than her. It cannot be less than or more than 10 years (8, 9, 11 or 12 years). That's why I feel it quite strange when I understand the meaning of that sentence: '...Ted is at least and probably more ten years older than me...'
Could you kindly tell me whether I have understood that sentence correctly?