Markings coming off bottles after 2 months

My baby is 3 months old. I have 3 Phillip Avent bottles. I used the 4oz new born bottle for a month. The other 2 is the 9oz bottle and both the marking are off. I only used that for 2 months. How can the markings come off after 2 months. These are poor quality markings. These markings are relevant and needed for making babies milk using the correct water and formula. I am really disappointed that I wasted my money on 3 Avent bottles.