about kog functional classification

Question

[Kurban]

hello to all， when i finished my kog classification i found that some of the functional classes pretty smilar in the result, e.g. [Amino acid transport and metabolism] and [Amino acid transport and metabolism, Posttranslational modification, protein turnover, chaperones]. and there are several others too.why these classes not catigorized to one class instead of two? could anyone explain to me the reason?