This doesn't just happen at the beginning of lessons, it's often within the activity part of the lesson too. Children are given a task and asked to complete it and some children find they missed the beginning of the lesson because they were flummoxed by the numbers and now can't follow through. If they did manage to follow the start of the lesson there are some children that still find the work difficult to follow as they are having to constantly interrupt the work they are doing to sort out the numbers. For example: when doing some multiplication with 2-digit numbers some children are having to stop half way through the sum to painstakingly work out the simple multiplications. (this is a rather extreme example to illustrate my point, hopefully nothing this extreme should be going on in any classroom!)
For those of you that are just here because your children can do some maths and enjoy it then again work on those mental maths skills, they can only make them better, happier mathematicians.