In the tradition of Albert Einstein let's try a little thought experiment. Let's go to the Grand Canyon and throw a perfect bullet spin pitch off of a cliff. The ball starts out with a perfectly horizontal trajectory and since it's bullet spin there is no Magnus force present. As the ball loses momentum and it's trajectory starts to change the gyroscopic effect keeps the spin axis horizontal. The ball continues to lose forward momentum and you might think that it will simply drop in an ever steeper trajectory, in a straight line down until it hits the ground. But this is not the case, as the ball begins to drop air flow over the ball increases from the bottom to the top and this generates Magnus force. From the perspective of the thrower if the ball has clockwise bullet spin the ball will 'curve' to the left as it drops. The steeper the drop the more it 'curves'. So a bullet spin pitch which started out dead straight ends up curving, in baseball it's the slider.The Magnus Force is proportional to the cross-product of the angular velocity vector and the translational velocity vector. Neither of these things are constant during the trajectory of the ball hence this force (a part of the force acting on the ball which makes it move) will not be constant. I would probably expect the magnitude of this Force to actually decrease during flight hence if this force produces a component which opposes gravity, which is pretty much constant, then "late movement" (in the direction of gravity) would be expected in this case. This is probably what you are seeing with a splitter, which has less backspin then a regular fastball. During it's whole trajectory it is going to drop more than a 4 seam fastball but there may be a bigger change (decrease) in the Magnus force during the end of the trajectory which causes it to drop more...not sure.
As a general statement, fluid dynamics is an inherently unsteady phenomena and the fluid dynamics of a baseball trajectory is no different. Hence the forces produced to move a ball during flight will not be constant. Knuckleball movement in baseball is the most obvious result of this unsteadiness..