It can always happen that due to unforeseen staff shortage someone else has to jump in who normally does not deal with KL flights. That's why.

Additionally, check-in jobs are (at least in the UK) learned by training on the job, so it can always happen that you deal with someone who has not had much experience yet.

Independent of this, in this job you have to know a lot about the different FFPs of all relevant airlines, including loosely affiliated airlines. On top of this loads of different baggage rules, maybe upgrade rules and loads of operational procedures, too, etc. And now imagine you have to know this for say 5 airlines that you regularly do the check-in for. It's easy to get confused with all this. And all this in a job where you receive just a couple of weeks of training and people do typically not have a tertiary education.

A good friend of mine once had this job at LCY, and, believe me, getting to terms with all these rules is not easy for the staff members involved. In particular as the world of frequent flyers is in no way related to their personal experience. (Otherwise they would have a different job.)

Finally, I am sure that KL has a quality assurance program for outstations. But I am also sure that it is very difficult for any airline to be in control of the details of what happens at dozens of outstations.

Of course, I am not writing this to justify rude behaviour or incompetence. But I find it important that we as privileged frequent flyers occassionally put ourselves in the shoes of the staff members that we happen to deal with during our travel. Given their particular "way of life" and the societal setting they live in, which includes comparably low salaries and high rents in the UK, their lives do essentially not revolve around us, even if we might be tempted to assume so and airlines try to make us believe so. And this is absolutely ok as long as they do their job properly enough to justify their (quite low) salaries. "Doing their job properly" means that there is also room for making mistakes. And I hope to have shown that, given the particular setting of staff at outstations, mistakes can happen more easily than also these staff member would wish for.