My guess would be because it's farther for the dope to travel through your blood stream before reaching your heart and then your brain, allowing for a certain amount of dilution within that volume of blood, easing the rush somewhat.
But yeah I've noticed that too in the past - that said, the one time I legit thought I was dying from a huge shot I was having trouble registering, all dehydrated and the like, when I decided fuck it, I was gonna hit one of this gnarly cluster of varicose veins on my left thigh. I registered no problem, even though it hurt like a bitch, but that was to be expected. What I didn't expect was for that crimson bloom in the barrel to be bright fucking red. Artery. I said fuck it, pushed, it burnt like hell but when I checked I was still in so I slammed it home.
The burn was excruciating, but I didn't care. This was about 90 units of straight syrup, and by about 40 my eyes were already strobing. I was sitting on the edge of the bath tub. After I slammed it down - by this point emitting this crazy ass ululating Hank Hill yell - I jerked the rig out my leg, threw it across the bathroom and fell back into the tub lol. Shit was intense.
Took me a few to collect myself and regroup from that one, and that was actually probably one of the most intense rushes I've ever had on meth, but it was also an exception to the norm.