Having the same number of injuries but more severe is conceptually bizarre.
How so?
It means they're playing the same sport in the same way generating the same number of incidents but those incidents are more serious potentially due to one change in variables - the material used to make the pitch.
It is, of course, still possible that it's due to another factor but since it's a statistically significant finding at this stage, and taken over five years, they've decided to investigate.