Highly Compensated Employee Rules Aim to Make 401k's Fair would be the piece that I suspect you are missing here. I remember hearing of this rule when I worked in the US and can understand why it exists. A key quote from the article:
You wouldn't think the prospect of getting money from an employer
would be nerve-wracking. But those jittery co-workers are highly
compensated employees (HCEs) concerned that they will receive a refund
of excess 401k contributions because their plan failed its
discrimination test. A refund means they will owe more income tax for
the current tax year. Geersk (a pseudonym), who is also an HCE, is in
information services and manages the computers that process his firm's
401k plan.
401(k) - Wikipedia reference on this:
To help ensure that companies extend their 401(k) plans to low-paid
employees, an IRS rule limits the maximum deferral by the company's
"highly compensated" employees, based on the average deferral by the
company's non-highly compensated employees. If the less compensated
employees are allowed to save more for retirement, then the executives
are allowed to save more for retirement. This provision is enforced
via "non-discrimination testing". Non-discrimination testing takes the
deferral rates of "highly compensated employees" (HCEs) and compares
them to non-highly compensated employees (NHCEs). An HCE in 2008 is
defined as an employee with compensation of greater than $100,000 in
2007 or an employee that owned more than 5% of the business at any
time during the year or the preceding year.[13] In addition to the
$100,000 limit for determining HCEs, employers can elect to limit the
top-paid group of employees to the top 20% of employees ranked by
compensation.[13] That is for plans whose first day of the plan year
is in calendar year 2007, we look to each employee's prior year gross
compensation (also known as 'Medicare wages') and those who earned
more than $100,000 are HCEs. Most testing done now in 2009 will be for
the 2008 plan year and compare employees' 2007 plan year gross
compensation to the $100,000 threshold for 2007 to determine who is
HCE and who is a NHCE. The threshold was $110,000 in 2010 and it did
not change for 2011.
The average deferral percentage (ADP) of all HCEs, as a group, can be
no more than 2 percentage points greater (or 125% of, whichever is
more) than the NHCEs, as a group. This is known as the ADP test. When
a plan fails the ADP test, it essentially has two options to come into
compliance. It can have a return of excess done to the HCEs to bring
their ADP to a lower, passing, level. Or it can process a "qualified
non-elective contribution" (QNEC) to some or all of the NHCEs to raise
their ADP to a passing level. The return of excess requires the plan
to send a taxable distribution to the HCEs (or reclassify regular
contributions as catch-up contributions subject to the annual catch-up
limit for those HCEs over 50) by March 15 of the year following the
failed test. A QNEC must be an immediately vested contribution.
The annual contribution percentage (ACP) test is similarly performed
but also includes employer matching and employee after-tax
contributions. ACPs do not use the simple 2% threshold, and include
other provisions which can allow the plan to "shift" excess passing
rates from the ADP over to the ACP. A failed ACP test is likewise
addressed through return of excess, or a QNEC or qualified match
(QMAC).
There are a number of "safe harbor" provisions that can allow a
company to be exempted from the ADP test. This includes making a "safe
harbor" employer contribution to employees' accounts. Safe harbor
contributions can take the form of a match (generally totaling 4% of
pay) or a non-elective profit sharing (totaling 3% of pay). Safe
harbor 401(k) contributions must be 100% vested at all times with
immediate eligibility for employees. There are other administrative
requirements within the safe harbor, such as requiring the employer to
notify all eligible employees of the opportunity to participate in the
plan, and restricting the employer from suspending participants for
any reason other than due to a hardship withdrawal.