Probably because you built a Data CD and not an Audio CD. Unless you used the Create CUE function, you didn't create an Audio CD but a Data CD. Data CD doesn't care how long the MP3's file are as long as the file sizes don't exceed the side of a CD. And 12 MP3's at 50 minutes each probably don't exceed 750 MB, the amount of data on a CD. As you said, the resulting creation was about 550 MB, which would fit on a CD. You said you used the "build" function, which would create a Data CD.
As you say, if you built an Audio CD, there would be no way to put all 12 50 minute tracks on one Audio CD. In fact, you could only fit 1 track per CD if you created an Audio CD.