The stock systems are `standard` --- you just happen to have found that they fit you nice and snug .... it`s just a matter of it being the ``luck of the draw``. The specification on stock systems are : Medium density, Freestyle, Graduated front, and 6 inch hair length. I think also the hair is slightwave, but am open to correction on that . The advantage is as you said --delivery time is fast. A custom system is to your own individual specifications, with flaps, density, colour, style. Custom can take up to 6 weeks or more, also depending on where you are located, it can add almost another week in the mail. If you are happy with stock, stick with that -- if you want things like flaps, poly perimeter, and different densities on various areas of your system, you will need to make a template and order a custom piece.
Well -done on doing your own cut-in -- I saw the picture you posted a little while back. Regards, Paul.