its with regards to the split in buddhism after sometime when buddha has died due to interpretation of rules. and after the split there are many other texts suddenly appears when buddhism spread northwards. those sects that accept those text are known as northern sch and those that reject the new one are the theravada. the difference if you were to ask me is mainly due to those extra texts and cultural differences. ahbidhamma is a very boring but highly organised form of teaching in buddhism with regards to analysis of mind and matter. it fact its more like studying psychology from the view of buddhism when you read it, it is the way buddhism classified different modes of thinking and how it view matters the nature of it.