The rosette pattern is a speciality of the offset printing process. It is a bit more difficult to explain why it exits, but I think this page summerizes it quite well. For detecting counterfeits, you need to know the pattern which is common for the card you are looking at. That is also the reason why I avoid posting pictures in this section. Even most fakes have a rosette pattern since they are printed in CMYK.
To add some trivia – WOTC used a software called Aldus PrePrint for the CMYK color separation during the pre-press process in the early days, which is the reason for the unique pattern/separation style found on old cards and especially the back.
Most of the time using a card from the same set and type works well but it gets more difficult if you are looking on older sets. Old sets have sometimes even different patterns for the same card. Also don’t mix up cards printed in foreign languages (i.e. compare ital. Legends to engl. Legends).
The only way to be 100% able to tell if a card is fake or not from the rosette pattern is by knowing the existing good ones for the exact same card or at least for the set.
If you cannot see a rosette pattern or you see a pattern in black lines or even white borders, the card you are looking at is 100% fake.