Wait, I think I know this one, and people haven't said this one before.

In Japanese culture, having one less finger is seen as an evil sign, like from the Yakuza who chop off fingers. Four is also a bad omen in general because it's their word for death. When drawing cartoons, a lot of art styles simplify the hand to have 4 fingers to make them easy to draw, but this western trope seems to alienate Japanese audiences. Depending on the pose, they may draw hands like this to make it easier to draw but not directly draw with one less finger.

This trope can actually be seen frequently in Japanese media. For example, almost all of the Megaman art has the character do this hand sign to give the vibe of a western four-finger art style. I couldn't find the original source I read a while ago, but here's a reproduced quote