Hello.
Everything at INZ works by managed queues. When your ITA was received, it would join the end of the queue of other ITAs, according to the date order. Each Case Officer has a certain workload - a number of cases that are theirs to deal with. The newly arrived ITAs have to wait till one of the COs has finished with some of their cases, and can take on some more. When an ITA is eventually assigned to a CO, it will again join the back of THAT queue for the CO's attention, as s/he takes the top file, does what s/he can actively do, puts that file to the back, works everything s/he can on that case, goes to the next, and so on.
There is no way to tell how many other people's ITAs were in the queue to be assigned already, on the day yours arrived. There is no way to tell how easy or difficult the cases already being worked in the office are turning out to be, which affects the time passing until COs are ready to take more files into their personal allocation. There is no way to tell if three experienced COs in that office might leave and need to be replaced, with the new COs needing training. There is no way to tell if there could be a flu epidemic affecting the whole building. This is why INZ cannot promise anything about timings, and it's never really a matter of 'just assigning' a Case Officer.