Powders do burn at various rates and the slower burning powders will take longer to achieve peak pressures. Remember pressure has to build to reach it's peak and this can be seen if plotted on a linear scale. So a slow burning powder in a short barrel won't have time to reach it's peak pressure resulting in lower velocities and the great fireball when the burning gases exits the muzzle. This fireball is also burning powder and sometimes all this powder does not get a chance to completely burn. Loads developed for the shorter barrels use a faster burning powder. Burn some gunpowder and you will see that it doesn't all go up at once, but actually takes some time to be completely consumed.
The .38spl is not at the bottom of an expansion rate for bullet makers. Remember the same bullet is used in both the .38spl and the .357 mag. If the bullet is made to expand to quickly then the velocity of the .357 mag can cause the bullet to blow up on contact. Remember that a bullet does not have to have a jacket. A fairly soft lead bullet expands nicely at low velocities and have always performed excellently in short barreled .38spl revolvers. The same bullet can violently expand in a .357 mag and have limited penetration. The new Gold Dot bullet is an example of a thin jacketed bullet designed to expand at .38 spl velocities in short barrels. This bullet is not used in .357 mag loads.