The answer is that OSHA doesn't penalize based on the number of injuries or their severity. It penalizes employers based on violations of standards and management's failure to implement policies to comply with these standards.
A fatality might be the result of one violation. A lesser injury might be the result of several violations. If OSHA cannot show that any management willfulness occurred that allowed the violation, then the maximum penalty is $7,000, regardless of how many people were hurt or even died.
Source: Health & Hygiene/ELB, Chapel Hill, NC, June 1998.