Joined wav longer than sum of parts

[InfoCynic]

I am trying to convert a 3-part series to DivX and I want to merge the parts together before beginning the conversion. I extracted the audio to 3 seperate AC3 files and then used Azid to get WAVs. These wavs are all normal and playback fine, and when I run lame on them for just a minute to test them, everything looks normal. So I joined the WAVs using CoolEdit, and the resulting WAV looks fine, the length is correct and it plays fine in Winamp. But when I try to run LAME on it, the number of frames is WAY higher than the sum of the 3 segments' frames (it's about 5-6 times what it should be) and the time estimate is too high as well. I let it run more than a third of the way through once and the output was longer than the entire joined WAV. I'm going to try to encode each segment seperately and merge the MP3s using a binary copy append, since I've heard that works, but it seems like what I'm doing should work. Ideas?

Thanks