From my experience, primary school teachers don't seem to be all that great at maths.
In my training (I did a 4 year teaching degree) I was once sitting in a maths lesson, where we were asked to work out 2 - 5. No-one on my table could work it out
. I explained it to them, then had a tap on my shoulder. Another fellow trainee wanted me to explain it to his table too. So I did.
In another lesson, I said that 100 / 8 is 12.5 and people looked at me like I'd done magic. I explained that it's a half of a half of a half (50, 25, 12.5) but they couldn't follow that.
I was a bit
that they had GCSE level maths. This was in 2007, incidentally. So not way back when.
So back to the classroom: lots of the teachers are not going to have fantastic maths themselves. What they do have is a calculation policy they are following. This will be broadly the same in most state schools. When I was at school, we learnt a method. Stick a number in, wiggle some bits around and hey presto you have an answer. Is it right? No idea- but it's an answer.
The new way that we teach number is to fully understand the value of it. So rather than 164 + 274 = carry the one (where's the one?) and add the 1 and 2 (there isn't a one or two- it's 100 and 200) etc we teach it as 100 + 200 + 60 + 70 + 4 + 4, then when children are absolutely solid on place value, you can use shorter methods.