iPhoto event labels question

I imported photos from an iPhoto library, made by iPhoto '11, into iPhoto '11. The event labels were were not preserved. Instead each event was labeled with a long series of numbers with a dash in the middle.

Why did this happen? What happened to the event labels? I went through and relabeled them. In the future if I want to import photos to a mac, I would like to know how to keep the event labels.

thanks, Kevin

Could you explain a bit about:

a. Why you needed to import one iPhoto Library into another iPhoto Library?
b. What method you used to do this
c. OS version, Mac model, etc.