Spectical lens quality

Bought Specticals with plastic lenses. Lenses scratch easily and had them replaced. Now need to be replaced again, First time was R400 rand and now they want me to pay R800 again for the same quality scratching lenses. My other Specticals before these ones are 4 years old and no scratches. Now Spec saver is telling me it is the same quality. Wont use spec savers again. They said that specs 4 u and spec savers uses the same fabrication to make glasses. So why is that the quality is so different