Am i allowed to guess?
The 1999 release will probably have been copies from the earlier digitised versions on CD. Whereas the 2000 version might have been from the original analogue recordings.

However, the question is whether there was any remastering of the 1999 recordings since the early released CD's were directly from anologue sources & thus the freqency response is different to the original (ie: sounds a little trebly.) Anybody who bought the very early CD's that were converted from analogue sources would know what I mean. The original analogue recordings were better. So really, for the 2000 version, they should have mastered from the earlier digitised versions & not the original analogue version. (That's if the original source is now in such poor condition.)