If something/someone "turns out to be" something, then it means that something else was previously assumed or predicted, but when the truth became clear, it was something different.
The weather forecast predicted it would rain today, but it turned out to be very sunny all day.
The new woman at work seemed really nice, but she turned out to be very nasty.
I thought my gold necklace was very valuable, but it turned out to be worth less than £50.
Student or Learner