Accio Prophecy!

[Arya]

No: HPFGUIDX 83890

I just started to wonder about this....Why did the Death Munchers 
not "Disarm" Harry to get the Prophecy sphere from him?  Why did they 
only attempt to Summon it (Accio) which allowed Harry to merely hold 
on to the ball to counter the Summoning Charm?  

It is apparent from CoS (no book here, sorry) that you can "Disarm" 
any possible "thing" or possible "weapon" from a person 
with "Expelliarmus".  Evidence? When the singing Love-O-Gram thing 
comes from Ginny to Harry in the hallway nad his bag rips, spilling 
his books out, including Riddle's diary, and Draco picks up the 
Diary.  Harry, gathering his things and then seeing Draco with the 
Diary commands him to "Give it here, Malfoy!" which of course only 
makes him want to check it out.  Harry then cries "Expelliarmus!" and 
disarms Malfoy, sending the Diary flying back towards Harry who then 
catches it.  
It appears to me that this is the same thing that the Death Eaters 
wanted to happen with that prophecy.  Sure, summoning may allow for 
greater control of the item in question, but still, I don't think 
we've seen a good counter to the Disarming Charm yet.  Certainly, it 
would have forced Harry to *let go* of the ball and would have worked 
much better than Summoning.  So, the question is--why didn't they use 
it???

Arya (Who might as well just give up all pretense of trying to appear 
like she's working while at work and just bring in her set of HP 
books, set up an Imperturbable Charm, and tell her coworkers not to 
bug her about worthless work issues ever again for fear of being 
hexed.)