1. Higher activity levels. Get serum injected into muscle, contract that muscle a lot and it gets pushed out into interstitial fluid and eventually into the blood, or directly into the blood if the needle went through a blood vessel on the way in. More guys go out and play basketball after getting the shot, more women sit around nursing their arm.

2. Larger blood vessels in arm muscle. More muscle, more you've used it, more likely for the injection to go into the blood directly.

That's my theory based on the question they'll never answer, which is what happens if the vaccine accidentally gets injected into your blood instead of muscle? The answer of course is you get blood clots and myocarditis and are likely to die.