Several employees struggled, but only one got fired: Discrimination?

Sometimes, multiple employees are guilty of the same infraction — but for whatever reason, the manager only fires one of them. Are companies in that situation always guilty of bias?
In one recent case, an employee was fired after several warnings about poor performance. She was placed on two different performance improvement plans, but failed to improve.
She sued the company for age discrimination. Why? She claimed that other, younger employees struggled just as much but were given more chances.
The company justified its decision, saying that the woman’s performance was lower than the other struggling employees — and as a more experienced worker, the company had higher expectations of her.
The court agreed, and the case was thrown out. Employees with similar problems and violations don’t necessarily need to be treated exactly the same.
Managers can prioritize their actions, and it’s reasonable to hold more experienced people to higher standards than others.
Cite: Faas v. Sears, Roebuck & Co.