Circular polarizers are not all built the same. A CPL is a stack of optical layers, and different materials and manufacturing methods can cause different amounts of unwanted light loss.

In an ideal polarizer, the unwanted polarization is blocked while as much of the desired polarization as possible passes through. In real filters, some of that wanted light is also lost through absorption, reflection, and inefficiencies in the polarizing and additional filter layers. Better materials, coatings, and construction can reduce those losses.

So two CPLs can produce a similar polarizing effect, yet one may transmit more light because it wastes less of the light you actually want. The difference is usually due to the optical properties of the layers and how efficiently they are made, not because the laws of polarization are different.

In short: similar polarization effect, different transmission efficiency.