The time in shipping could be part of the difference.
The weather might make a difference too. Being shipped in hot weather, or cold weather, or moderat weather, can make a difference in how much the chicks are stressed.
Different breeds, and different individual chicks, could also have more...
That is the usual advice, but I prefer a simpler method (similar to what @Ridgerunner does).
Make one corner be 90 degrees or warmer.
Make the brooder big enough that they have plenty of cool space.
If needed, add even more cool space.
Leave the warm area alone, always the same temperature...