I stole this from another board (not exactly webMD), but I would assume it would be something related to this:

Chemical = Formula = Molecular Weight = Mg of Testosterone
Testosterone (no ester) = C19 H28 O2 = 288.4mg = 100mg
Propionate = C3 H4 O = 56.1mg = 83.72mg
Cypionate = C8 H4 O = 124.2mg = 69.90mg

This is test, but I would assume it is the same. Different esters have different weight, therefore you are actually getting different amounts of Tren depending on the ester.