I'd like to think that I set out to keep up my IAM test standard each time I get on my bike.
I'm sure you do aim to do that but as with any skill you care to think of, if you never do any refresher training or re-testing, the chances are that in 5 years time you will have picked up a couple of bad habits without being aware of them. That's just human nature.

If you want to reach a higher standard or check you haven't picked up any bad habits then you have two choices.
You could voluntarily put yourself forward for the IAM re-test or the IAM special assesment.
There is nothing stopping you doing this any time you want. All I'm saying is that the majority of IAM members do not put themselves forward for re-testing.
At Rospa it's mandatory so you don't have a choice if you want to maintain your qualification.
Not only is it mandatory but it's also free and scheduled which are important factors in making it happen.

Many IAM members think about doing re-testing but don't get round to it (and I am included in that group).