Although I still have material to read, my current thought is the following: Trotsky was closer to the Mensheviks than to the Bolshevik faction; or if you prefer, his approach was situated somewhere between both currents. If he joined the Bolshevik faction, it was out of pure pragmatism.

If you read Lenin in ‘What Is to Be Done?’ or ‘The State and Revolution,’ you will see how he criticizes the Russian social democrats of that time, from the opportunist branch (referring among others to the Mensheviks and similar), for wanting to collaborate with the government and divert the proletariat from the path of revolution. Stalin does nothing more than follow the path started by Lenin.

Hence, the ‘orthodox’ thought of Marxism sees Trotsky as a revisionist/reformist.