It's starting to come together in my brain. There appear to be two parts to this. One is converting 24 fps film to 25fps PAL video. The second is converting between analog NTSC 30fps and PAL 25fps. (I might need more understanding of this second part). Both result in exactly 4% difference.
But when doing computer conversion between these fps the converter has no-o-o-o pro-o-o-o-blem maintaining the length.