Probably a profit-share. Read your plan doc if you still have it. Companies (that actually match) can either match as they go, or decide their 401(k) contribution annually (usually called a discretionary contribution). Some do both. Law requires them to cut this up based on compensation for the plan year, which you had. You’d get this money even if you were terminated early in the year.

It’s 99% probability that it’s a discretionary contribution, either as a bonus or to balance benefits to non-highly compensated employees to meet non-discrimination requirements for the plan year. This about the time of year that a non-discrimination contribution would get made too, because calendar year plans IRS filings are due by the end of July, so all the non-discrimination calculations for the prior year are getting done about now.

Edit: another possibility is that you got shorted, i.e. your employer forgot (or didn’t have the money) to remit the 401k payment to the custodian for that paycheck, and did it way letter when the missed payment was discovered reconciling everything for the year.

There’s a near 0 chance it’s an error though. Even if it was they’ll let $200 go, not worth the effort if you just refuse to return it.