If your wheelchair is being provided by social services they will ensure that you are given the correct wheelchair..
If you are purchasing it yourself, then the mobility shop will go through with you what your needs are and to match a chair that suits you..
Both we also measure so that the wheelchair isn't too big or too small...
The only time chairs are wrong, is when somebody decides to purchase one themselves, either via mail order etc without any advice from the supplier... Or if the wheelchair is a short-term loan chair where it tends to be 'pot luck' situation with the belive that well, you won't be using for long..