What is the logic behind attributes saved in target

I am using phoneCall entity, first I used my logic in phoneCallSetState and got all the data I needed, for exampeI have a costume field which called category, and I changed it when I created a new record and then I could get the category value (in c# code side) but later I took the same code  and added it to my phoneCallPostUpdate and after I changed the same field - category (but now under postupdate msg) I got NULL from the same field. I used postImage to get the data eventually but I need an explanation why I could get the data from the same field I changed in SetState but got NULL in PostUpdate?

Thanks