I was thinking the same thing. I think math lends itself to this because the work is so esoteric that often the advisor doesn't even completely understand the work, much less can claim credit for it.
I think the size of labs is more important. Bio and Chem labs -- especially the best ones -- are often freaking huge. There's no way the PI actually has time to meaningfully contribute to every paper when they are bringing in enough money to support dozens of full-time positions.
In math, the group sizes are way smaller. A PI might only have 1 or 2 students.
In math, the group sizes are way smaller. A PI might only have 1 or 2 students.