Get location of dropped item doesn't work

Hey there!

I need to find out the location of a dropped item after specific intervalls.
I used a Sync. Timer, calling my run() every 20 ticks / 1 second to print out the current height of the item:

Code:

double y = item.getLocation().getY();
System.out.println("Y: " + y)

My ground is at 71.5 (there's a half-slab / step)
I dropped the items from a height of 100

In most cases, the output looks something like this:
100
81.9264
71.5
71.5
71.5

But sometimes, this happens:
100
81.8203
71.73333
71.73333
71.73333
71.73333

And although the item is (viewed from ingame) clearly on the ground, the location data seems to be stuck at a false height. Sometimes the Y-value is even more extreme and e.g. stuck 10 blocks above the ground, although the item IS (in real) on the ground!

What did I make wrong or why doesn't the location update sometimes? :l

- Cedi