It has to do with the direction of the exponential.  In the real world, when people buy stocks, they try to buy ones where the exponent is positive r = A*exp(x/t), like the red line.  But mining is intrinsically a negative exponent process r = A*exp((1-x)/t), like the blue line. So MPEX is winning because they carry red line companies, not blue line companies.

Also, no ponzi.