Wrong. The Variance of both specs is big enough that fire can pull ahead with good RNG on the Combustions in single Target. In Fact, with both specs having their best possible RNG, fire will come out ahead in single target with current max gear. So you can essentially say, that fire has the higher single target DPS potential of both specs but arcane wins on average. On
www.simulationcraft.com you see the average among 25000+parses, if u check the variance you will see that fire actually can reach bigger dps with enough RNG. But your claim, that you will never see fire ahead of arcane is both practically (see warcraft logs oregorger/gruul with fire pulling ahead) and theoretically wrong.
Simulations are one thing, current oregorger logs are another thing:
https://www.warcraftlogs.com/rankings/7#boss=1696
#1 parse is a fire Mage and on Gruul Fire and Arcane are pretty much equal.