They're somehow assigning more weight to the negative rating then the all the others. There's perhaps some value here in doing so, since there might be a legitimate complaint, but often we need to discard this one form as an outlier.
If an attendee gives a negative rating (one or two on a scale of five), then they should provide related comments. For example, if The speaker didn't present well; then how exactly? Was it the volume of his voice, the quality of the slides, or perhaps the inappropriateness of an opening joke?
"Do not seek to follow in the footsteps of the wise. Instead, seek what they sought." - Matsuo Basho