I think adding packing peanuts may depend on several scenarios, rather than a particular hatchery.
Number of chicks ordered and the box needed...may be extra room so they fill it up.
Time of year/applicable weather conditions...if it's real cold they may add more for warmth.
Number of chicks hatched that day compared to order to fill....they may have extras so they stick them in.
(this may be why those 'bonus chicks' are exotics'.)
Glitchy hatch with maybe some weak chicks...add extras cause some may not 'thrive'.
Hard to say why and what those extras are....lots of possibilities.