Need some clarification on depth and stepover

So I’ve been doing some aluminum milling and I usually do a 5mm deep with 1.2mm stepover, and sometimes it works sometimes it chatters like crazy
But yesterday I programed a face milling with 5mm deep 5.6mm stepover, and Holly crap it cuts like butter.
So, how come that a smaller stepover is more unstable then a bigger one.
To be mentioned that it only works in climb direction
The feed was 600mm/min on both setups

Assuming the same tool, the increase in tool engagement can cause less chatter. You can also get into resonant frequencies of your machine that can cause chatter. You don’t mention your spindle speed…you could increase the speed to decrease chipload which can also affect chatter. Adaptive/trochoidal toolpaths help with more consistent tool engagement.