The baseball players union is more than curious about why certain teams, such as the Rays, aren't spending more money.

So the union filed a grievance to Major League Baseball against four teams — the A's, Marlins, Pirates and Rays — claiming they have failed to comply with rules of how they spend their revenue sharing money, the Tampa Bay Times has learned.

Teams are required to spend their revenue-sharing money to improve the on-field product, according to terms of the collective bargaining agreement, though not necessarily on their major-league payroll.

The Rays are considered one of the biggest revenue-sharing recipients, getting what is believed to be about $45 million a year.

MLB responded with a statement to the Times: "We have received the grievance and believe it has no merit."

Rays principal owner Stuart Sternberg said he was "genuinely surprised" that the grievance was filed and that the Rays were "beyond what compliance is" with the rules.

"We've run our organization in a very open, transparent fashion since the day I've come in and try to prepare people for what we're doing," he said. "I don't know what happens from here. It's uncharted territory for me, and I would imagine the other teams as well. But if it wants to be explored, or needs to be explored, I don't get it. How's that?"

Union spokesman Chris Dahl confirmed this morning that a grievance had been filed Friday but offered no other details.

The complaint covers the 2017 season and the current offseason.

It comes after an unprecedentedly slow winter market for free agents, with speculation from players and agents about collusion among teams, and some suggestions that the union leadership may have been complicit for agreeing to some of the terms in the most recent CBA.

It is not clear what the result of the grievance could be even if the teams were found to be in the wrong, as the union wants some form of compensation for players that were not signed.

In January, the union notified the commissioner's office that it was concerned about what the Marlins and Pirates were doing and asked for answers.

"We have raised our concerns regarding both Miami and Pittsburgh with the Commissioner, as is the protocol under the collective bargaining agreement and its Revenue Sharing provisions," the union said in a statement, as reported by USA Today. "We are waiting to have further dialogue and that will dictate our next steps."

At the time, MLB responded in a statement to USA Today: "We do not have concerns about the Pirates' and Marlins' compliance with the Basic Agreement provisions regarding the use of revenue sharing proceeds. The Pirates have steadily increased their payroll over the years while at the same time decreasing their revenue sharing. The Marlins' ownership purchased a team that incurred substantial financial losses the prior two seasons, and even with revenue sharing and significant expense reduction, the team is projected to lose money in 2018. The Union has not informed us that it intends to file a grievance against either team."