The most likely explanation is a technical glitch.
In order to allocate to classes, it's necessary to
1a. Export an allocations file, put Ts in the column (for true), save the file, upload the file, remember to set it to overwrite existing class memberships from the effective date and run the update class memberships routine. Whoever is either copying from a spreadsheet - which can have typos or errors - or putting Ts in the columns direct can make an error. Or they could have a really helpful email or five with Jess A, Jessica A, JezA, JesikaAA and Ajess featuring in those emails.
OR
1b. Manually click each kid's group box in the curriculum management module whilst flipping between spreadsheets/emails, save and run update class memberships.
This is assuming that there hasn't been a new timetable that needs to be applied first, as that adds a whole new level of complication and opportunity for things to go wrong.
2a. Then there will be a flurry of emails where people are asking that JessA and JerramyB aren't in the same classes as there has been a few problems, that JamieC and Jolene are moved to class 3 because that's got a TA in there, JackR has broken his leg in six places so needs to be in the class that isn't up five flights of stairs, the Maths class determines the science and tech ones and although a standard class can accommodate 33, the tech block can only fit in 25 so a whole bunch need to be moved. And now JuneF's mum has found out that she's in a class with AprilG, she now needs to be shifted across bands in order to ensure that they never meet in a classroom environment at any point following some very unpleasant events over the summer holiday between her, June's Dad and April's Auntie.
2b. There are a bunch of leavers and joiners around the end of September who have also needed to find the KS2 data for and put in for CAT4s. This means another go around of the updating.
2c. The roof in one of the Maths classrooms is leaking. The intended 6 classes has therefore shrunk into 5 and at the same time, it's been decided that actually, one of those classes needs to be a bit smaller than the others so the allocations need to be done again.
There are many pinch points where something can go wrong completely aside from it being a deliberate decision of staff or down to a kid deciding they don't want to stand out, not speaking up when taking online CAT4s and the screen has frozen on the quantitative reasoning set or behaving in a way that means their assessment scores are significantly lower than their actual ability.
In short, there is absolutely no harm at all in checking and it may have absolutely nothing to do with inadequate assessment and everything to do with it being a seemingly simple but technically complex process to create a new timetable. I prefer having a moveable structure in place from the outset to minimise the opportunities for problems, but sometimes you just have to deal with what you've been given.