Same velocity but different drop rates.?

A difference of 40 fps is nothing in commercial ammo and that applies to not only velocity differences within the same brand, but also difference of ammo from the same box. The muzzle velocity on the box is only an approximation. Depending on the quality of the manufacturer, a variation by as much as 40 +/- FPS is possible.

As for a bullet drop of 6 inches at 100 yards, there could be many factors that could cause this including you, temperature variations, and how clean/foul as well as the temperature of your rifles' barrel.

A bullets trajectory is not determined simply by velocity and mass of the projectile. For example, the construction of the bullet and the weight/mass distribution of the material and the surface of the bullet can mean it experiences more or less aerodynamic drag - this is refer to as the ballistic coefficient (BC) of a bullet. A high BC means the bullet more efficiently cuts through the air and therefore loses velocity (decelerates).

d ifferent bullet shape maybe?  rounded nose hollow point or a flat nose semi wadcutter are most common recommended  for under tube magazine carbines in .357 or .44 caliber. different drag factors. Powder burn rate for revolver may vary- a load for a snubby to plain meant for 2.5 to 4 inch 'low flash' use may test faster from revolver barrel-but will have burned completely at 8 inch while slower tested powder that has some flash yet in a Ruger Black hawk will still be burning at 14 to 18 inch of barrel in carbine.  their are few .357 rounds NOT recommended for use in carbines with under barrel magazines- pointed 'road blockers' meant for police use shooting through car bodies, very bad to load in under barrel magazines since pointed nose- sometimes iron cored- rests against primer of next round. Pointed lead bullets in .357 exist- most of boxs are marked for Thompson center or single shot rifle use- they do have good velocity, range and different drop factors.  for your use pick the ammo with smallest group 100 yards and sights adjusted, learn range estimation and mid range trajectory 

Ballistics charts for .357 Magnum Rifle usually post a muzzle velocity of 1,850 fps, with a muzzle energy of 1,175 foot pounds.

The figures are illusory. 

(A) The listed velocities on a box of handgun ammunition are usually those for the handgun, not a rifle.  So you might see a velocity of 1,200 fps printed on the box

(B) Only a chronograph will give you the true velocity, and these velocities are usually going to depart from the velocities listed on the box.  In addition, if Winchester and Remington have two loads with 158 grain bullets listed at 1,200 FPS, for example, don't expect the values from both brands when measured by a chronograph to give near identical numbers

(C) Rifle velocities are likely to be at least 300 fps faster than handgun velocities

(D) The velocities that are printed on box are the average velocities the manufacturer has noted by firing the ammunition through a TEST BARREL that might be of any given length.

(E) A ballistics table might list a 158 grain .357 Magnum to develop 1,850 fps muzzle velocity.  Expect the REAL velocity to be as much as 200 fps slower.

(F) Since the REAL velocities are not likely to match the listed velocities, neither are the trajectories going to be identical

(G) Sometimes you find that even if everything seems reasonable "equal" between two brands of ammunition, some brands just may not be all that accurate or give the anticipated point of impact when fired at a target

The numbers on the box are typically pretty optimistic, and different companies may use different barrel configurations for their chronograph testing. 

Put your faith in the Universes's gravitational constant. It is more reliable than the velocity information printed on the back of the box.

Also keep in mind that a faster round will leave the barrel sooner before the barrel has risen as much from recoil. Recoil has to overcome inertia and so it is not instantaneous.  

Ignore the box and measure them yourself. So many time the stuff inside is slower than the marketing