Rulings by judges who ignore tougher sentencing laws are a case of the "tail wagging the dog," according to a federal lawyer who is appealing a Nova Scotia Court of Appeal decision to the Supreme Court of Canada on behalf of the federal government.

The issue is whether judges should have the discretion to give people in jail more credit for time served in custody prior to sentencing.

The Truth in Sentencing Act — a key component of the federal government's tough-on-crime strategy — prescribes a 1:1 ratio for credit for time served. The Nova Scotia case hinges on a trial judge who gave 1.5 days credit for time served in pretrial custody.

David Schermbrucker of the Public Prosecution Service of Canada offered a preview of the arguments the federal government will use as it challenges judicial rulings from Ontario and Nova Scotia at the Supreme Court of Canada next week.

"Our position before the Supreme Court is, essentially, that doesn't make sense," he told CBC News.

"Parliament would not have enacted a rule where the tail is wagging the dog. Everybody is getting the exception and nobody is getting what we say is the presumptive rule."

Schermbrucker, a Halifax-based Crown attorney, is appealing lower and upper court rulings in the case of Level Aaron Carvery, a Halifax man who spent months in jail prior to sentencing for drug offences.

Anne Derrick, a provincial court judge, reduced Carvery's sentence by 1.5 days for every day spent in custody prior to sentencing. Derrick's ruling was upheld at the Nova Scotia Court of Appeal by Justice Duncan Beveridge.

Both judges noted the Truth in Sentencing Act allows a judge to deviate from the maximum credit of 1:1 and offer an enhanced credit "if the circumstances justify it."

'What's at stake is fairness'

In upholding the decision granting Carvery enhanced credit, Beveridge used the hypothetical case of identical twins charged with the same crime — one gets bail for six months while the other is in custody for six months.

Both twins plead guilty and get sentenced to 18 months imprisonment in the hypothetical case.

Beveridge said the twin who was granted bail would serve 12 months of the 18-month sentence and then be released, by law, after serving two-thirds of that sentence.

The twin not granted bail, on the other hand, would get six months credit for time in custody prior to sentencing — that means the sentence for this twin would be 12 months in jail. After eight months — two-thirds of the 12-month sentence — the person would be released.

That means the twin who was granted bail would serve 12 months in jail and the twin not granted bail would serve 14 months in jail.

Beveridge called that situation unfair.

But Schermbrucker said routinely granting enhanced credit guts the intention of the act.

"Our response is if you take that approach there's nothing left of the normal ratio of 1:1 because everybody is going to get enhanced credit," he said.

Schermbrucker's legal opponent at the Supreme Court of Canada is Halifax lawyer Luke Craggs, who represented Carvery.

"What's at stake is fairness for people who have not been granted bail," Craggs told CBC News.