Last week, Spanish Prime Minister Mariano Rajoy gave investors and analysts a pleasant surprise, announcing that his country's budget deficit had fallen to 6.7 percent of gross domestic product in 2012, far below the European Commission's estimate.

Unfortunately, the lower number is probably wishful thinking on Rajoy's part, because he excluded the costs associated with recapitalizing Spain's banks. The European Commission's estimate was much higher, at 10.2 percent of GDP, because it included them.

It isn't clear that Rajoy is right. Ireland, which also saw a property bubble burst topple its banks, was forced to account for its bank bailouts in the official figures, pushing up Ireland's overall deficit up by 3.7 percentage points to 13.1 percent of GDP in 2011.

So why has Spain been allowed to omit the bank recapitalizations from its deficit when Ireland was not? A cynic might argue that it's because including these costs would push the Spanish deficit up to levels so high that investors would dump their Spanish bond holdings, which policymakers would like to avoid.

More likely, it is because European Union policymakers have agreed in theory to mutualize bank recapitalizations retroactively once the European Central Bank has been established as a single supervisory mechanism by March 2014. This plan was not in place back in 2011 when Ireland had to recapitalize its banks.

Retroactive bank recapitalizations are far from a done deal, however. Just last week, the head of the EU's bailout fund Klaus Regling highlighted that several core euro area countries were not on board with the idea. Germany and the Netherlands in particular have raised concerns about mutualizing bank recapitalizations.

If these policymakers continue to resist using the EU bailout fund to retroactively recapitalize banks in the euro area, then Spain may have to adhere to the same accounting rules that Ireland did. This would push Spain's budget deficit for 2013 roughly four percentage points above its target for the year.

Investors might then question Spain's ability to control its deficit and begin to shun Spanish debt. The Spanish government would then be forced to implement ever more austerity to catch up to its fiscal targets, pushing the Spanish economy further into recession.

Investors are happy now about Spain's 6.7 percent of GDP deficit number for last year. Don't be surprised if this figure later gets revised upwards to less felicitous levels.

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