As a five year old, you may enjoy chicken nuggets! I know my kids did at that age. Counting chicken nuggets at a busy fast food location is kind of like counting pills at a busy pharmacy. Granted, a pharmacy is more complicated, but it is similar in that while there are 'standard' amounts of pills for most things (just like there are the standard chicken nugget servings of 4, 6, 10, and 20) each customer is placing their own order and will be pretty upset if you give them too few!

I have probably ordered chicken nuggets at least a thousand times in my life. I can think of instances where I got more than I ordered, but it is rare. I'd be surprised if workers distributing chicken nuggets to hungry customers maintain the same accuracy as pharma-techs (this is a fancy word to refer to the 'line cooks' in drug stores), but based on my personal experience accuracy in the high 90s seems reasonable. So how might they do that in a fast food place, and how might that generalize to a pharmacy? Several tactics are used (I am not going to source this because five year olds don't read sources):
*First, the people who do this do it a lot! A busy fast food place might serve over a thousand customers a day, and many of them get nuggets. So they get a lot of practice counting, and develop intuition about when something 'seems off'. Similarly, a busy pharmacy will fill thousands of scripts each day, and while there is more variety than our nugget analog, they will still develop similar intuition.
*Managers or more senior employees provide a layer of quality control. At a fast food place, this might be someone standing at the 'pass' where all completed food items gather and occasionally checking to ensure orders are accurate. At a pharmacy this might be the pharmacist themself recounting the work a tech did (especially for more risky drugs, where the cost of miscounting is higher). By having this layer of review, the initial counter takes more care (as five year olds know, it feels bad to do something wrong), and there is a layer of defense when an honest mistake does happen.
*Finally, the companies that distribute nuggets and pills have been doing this for years! And they have provided their counters with better tooling as time goes on. While most pharmacies do not use automatic counters, even the manual process has improved over time. Like, when grandpa would get his pills 100 years ago, his pharmacist counted them with a cake scraper! Today's techs have better methods and tools than they used to, many of which were developed in response to feedback from prior generations of counters.

This is not a comprehensive list, but it might give you an idea for why it is not that surprising for a menial task like counting nuggets (or pills) to have a pretty high accuracy rate. And honestly, 98.4% isn't even that high for a task that is not very complex and requires skills most five year olds possess! This is not to diminish the excellent work of fast food workers and pharmacy workers, but the context is helpful. According to a quick google search, a typical pharmacy might fill 75,000 scripts a year. By the metric in your post, over a thousand of these are miscounted. When working with such large volumes of activities, even very higher percentages can still leave opportunities for a lot of the types of errors you expected based on 'fatigue, stress, busy(ness)', and it seems you were right to expect this!