If you have a monthly budget of £650 set, I believe your Left to Spend should say £0 after the spending pattern you describe. I believe it should only say £10 if you don’t have a monthly budget set.
Not sure where the £70 left to spend comes from (still trying to get my head round this tab myself), but I wonder if you have £70 worth of Excluded transactions (bottom of summary screen)?
It does do this when you don’t set an overall budget. There are two different modes of operation for the Summary tab depending on whether or not you have set a budget. See my previous post, but I also managed to find another post which makes it clearer: