Donald Trump says he's worth $10 billion, which basically nobody believes. But even low-end estimates that think he's not nearly a billionaire concede that he is, in the grand scheme of things, a very rich person. After all, he's got his own jet. But as Aaron Elstein writes at Crain's, Trump's New York City property tax bill released over the weekend also shows he benefits from a tax break that's supposed to be limited to middle-class homeowners.

Specifically, Trump gets benefits from the New York state School Tax Relief Program (STAR) that are supposed to be limited to married couples who earn less than $500,000 a year. For Donald and Melania Trump to be receiving such a break seems like either a mistake or an abuse. Indeed, when Crain first reported on Trump and STAR in March, the Trump campaign said it was an error and a spokesperson for the mayor's office agreed.

But what kind of error, exactly, remained mysterious, since the city's finance department says it verifies every STAR application with the state tax authorities.

Today's report, meanwhile, confirms that Trump is still getting the tax break. So whatever error there was evidently hasn't been fixed.

Alternatively, maybe Trump is still getting the tax break because he is, in fact, eligible for the tax break. The $500,000 limit is a ceiling on taxable income. So if Trump is somehow pulling in millions a year but exploiting tax loopholes to drastically reduce his burden, that could make him eligible for yet another tax minimization measure.

Indeed, David Johnston's old book about Atlantic City, Temples of Chance, reports that Trump paid no income tax at all for several years in the early 1980s — back when he had to disclose his returns to secure gaming licenses from the state of New Jersey. The main tax minimization strategy he used back then was sharply curtailed by a 1986 tax reform law passed by Congress and signed by Ronald Reagan, and Trump complained quite vocally in the 1990s about his increased tax bill due to the law.

Since 1976, all presidential candidates have disclosed several years' worth of tax returns to allow their financial practices to be publicly scrutinized. If Trump did the same, it would be relatively easy to clear up whether he's eligible for the STAR credit and, if so, why. But he's refused, and so the mysteries persist.

Watch: How each candidate's tax plan impacts your wallet