Scoping Sirius' Curious Car Count

"It makes sense for us to do it when we have two parts of the transaction together," says Frear. "It's more symmetrical to take in the cash and record the expenses at the same time."

Sean Butson of Legg Mason has been one of the first analysts to highlight the impact of new cars on Sirius' subscriber tallies. Butson says it's necessary for analysts to back out unsold cars from actual subscribers when comparing the two companies' growth rates.

The bigger issue, says Butson, is that if a car goes unsold or if a user doesn't activate the service after the yearlong free trial, they don't immediately come off the subscriber count.

"To show up as a Sirius disconnect, it could be as long as 17 months," says Butson, who has a buy rating on Sirius and XM.

Stretching definitions to fuel Wall Street's growth expectations can help give you an edge in a close race, but some observers say it undercuts a company's credibility.

"It may be legal," says the former Sirius investor, "but at the very least, it is slippery as hell."