Google Analytics - different number of users in Real-time and by device category

so i want to understand this so in google analytics sometimes the numbers of users in last 30 minutes and user by device category are different so for example here i can see user by device category you can see 15 on mobile to on desktop so in sum it should be 17 right but users in the last 30 minutes it says 13. so that's kind of interesting to know why there is this kind of difference um and why is that happening so that the sum of the users by device category is different to the users in last 30 minutes plus matrix are in the last 30 minutes here you can see also people users by page in the last 30 minutes so for example if we sum up here so that will be 5 15 so 15 and here is 14. now it's six so it's kind of a bit strange uh maybe there is some kind of caching or some kind of issue which i don't know um so uh yeah i don't know any suggestion why that is happening that users by device category are different to users in real time it's happening on google analytics app on iphone so just interesting to know your opinion