UPS handles the fulfillment for Sprint handsets (including returns), they are not just the shipping carrier. Their system will assign the correct shipping product to meet the delivery commitment set in the order when it is transferred to them. Just like Amazon says 2-day shipping and generally my orders come via ground or even some USPS product.
Now, why Sprint has to use "Overnight Shipping" is unknown to me. They should just say 1-day shipping, which refers to the commitment not the specific product used by the carrier.
So the priority being location makes sense depending on what they have to ship out? I don’t know much about this so I’m pulling at straws but it’s the only logic I’ve been able to apply to this.