Can't speak for Mrz but for me when teaching my own dc I know my dd inside out.I know when she can't be arsed and would rather be doing something else. She's just not into maths,far more into literacy. When you know your child isn't doing their best it's hard not to loose patience which I really don't want to do so I back off when things start to kick off.
When you teach classes you're aware of what trying your best,putting your all into something is.I can sniff out half heartedness v quickly and your own child is far more likely to do that.It annoys me when you've taught kids of varying abilities who put their heart and sole into lessons you've prepared and you have your own child in front of you trying it on.
Also I'm aware they work hard at school,it's her free time and she's 7 ie I feel kind of bad anyway doing extra maths with her.
When you teach in school kids are fresher,they're there for a reason,they totally respect you(dd respects me but at the end of the day if she wants to examine bugs in the fruit bowl and chat about ponies she will because I'm her mum not her teacher)and keep to class rules and the behaviour you'd expect in school. There are very few distractions in school.
Then there is the simple fact dd and I are very similar ie we can clash big style.I see me when I teach her.
DD has 2 brothers who love maths and just seem more mathematical. When her brothers are off outside you get resentment re doing maths before you even start.
Being gifted at maths and teaching maths to a gifted child is a whole different ball game to teaching a child who lives in fairy tale land and would far rather be outside up a tree than discuss near doubles. Believe me it's a whole different game.I have a child who nags to do homework so I've experienced the difference.