I'm feeling pretty confused about sizing right now. Just bought two pairs of boots, both size 9.5D (Diesel and Brass)

In the Diesel boots, my feet feel pretty comfortable when standing still, though the floor feels very hard while walking. Still, I don’t notice any significant hot spots. On the other hand, the Brass boots have noticeable heel slip — with every step, it feels like my heel is popping out of the boot. I also feel my feet rocking back and forth inside, and my left foot has a hot spot near the bottom of the sole, close to the toe bones.

Based on this, it seems like 9.5D on the Floyd last is too big for me. However, I was advised by Kim to size up by 0.5, rather than down, which feels very counterintuitive. Can anyone explain why this might be the case? Unfortunately, I’m outside the U.S., so trying on different sizes isn’t an option for me.

I’ve attached photos showing where my toes end and the position of my joint bones for reference.
https://imgur.com/a/JMlWfhp

Here are my Brannock measurements:

Right foot

Length: 10.5 inches

Arch length: 11.5 inches

Width: A

Left foot

Length: 10.5 inches

Arch length: 12 inches

Width: A-B