There are at least three makes/models of 510 cartos: Boge (silver threads), SLB (brass threads), and the new Joye cartos (these require a bit more tinkering to fill). And there are several makes of atties, too. Resistance is not the same across different makes, meaning that some will produce more and warmer vapor than others. Also, what kind of filler are you using in the cartridges? If it is the stock polyfill, it does not wick well, and produces less vapor/flavor/th than a modified filler, such as blue foam (Marineland) or fluval, or ptb, or Highping, or....
There's no simple answer as to why you are getting better vapor from a carto than from an atty/cart. More info on the makes, filler, etc., might get us closer to an answer. I get much better vapor, most of the time, from an atty/cart than from a carto of any make.