if your method works keep doing it as its your product....
not an exact scientific answer why but my guess is as you heat/ smoke for longer periods of time the moisture and the casing "tenderness" losing the extra water attribute to the chewy bite, ive tried several methods and poaching seemed to keep the moisture locked in when using the mahogany collagen casings....
Personally anymore i use cellulose casing and everything i do is skinless because its very difficult to get the right tenderness that im used to commercially as i suspect they use a steam cooking cycle to maintain tenderness and nice shape to the meat sticks not the wrinkled look..... if i want a snack stick that has a snap or bite to it im using sheep casings, there no better alternative to sheep casing in my opinion