Yeah, that can be confusing. At low levels, the differences between modifier values between ATs are smaller, making many of them feel very similar as they are just starting out. As the game progresses, differences in modifiers for things like hold/immobilize/fear/etc. duration, buff magnitudes, hit points, melee/ranged damage, etc become more pronounced. This is primarily what gives - for example - a Tanker a different feel than a Brute or Scrapper.