As discussed in this investopedia article contributions may be limited for Highly Compensated Employees (HCE). This article discusses the Actual Deferral & Actual Contribution Percentage Tests (ADP/ACP) which are used to determine limits for HCEs.
However, in a unionized company, there can be other considerations . Union Bargaining agreements can override the plan terms, and cause a reduction in the contribution limits. The article "Challenges of Retirement Plans with Union Members" says:
Retirement plans at organizations where the workforce is largely unionized have the unique challenge of being obligated to negotiate plan terms with the union. It’s a process that can be very burdensome, experts say, but also very rewarding.
When a retirement plan is run at an organization with a union, the collective bargaining agreements trump the retirement plan document, as specified by the National Labor Relations Act, according to David Kern, partner with the labor and employment practice and chair of the National Labor Relations Act team at Quarles and Brady LLP, in Milwaukee. “Any changes to the plan need to be bargained with the union,” Kern says.
Retirement plans at non-union organizations typically have a provision that the employer has the right to modify the plan at any time, says Amy Ciepluch, chair of the employee benefits and executive compensation team at Quarles & Brady. At a union shop, however, “that type of language would not trump what a collective bargaining agreement guarantees. The collective bargaining agreement would govern.”
However, the federal limit on total retirement contributions is unchanged. So an employee limited in 401K contributions can increase contributions to an IRA or other retirement account.