The game won't pay attention to either the ArmorDropChances or the HandDropChances. I used the command:
/summon Skeleton ~ ~1 ~ {SkeletonType:1,ArmorItems:[{},{},{},{id:skull,Damage:3s,tag:{SkullOwner:kingplayer855}}],HandItems:[{id:blaze_rod,tag:{display:{Name:Fire Staff},ench:[{id:20,lvl:10}]}},{}],HandDropChances:[0:1.0f,1:1.0f]}
You have to add the Count: tag to your items, items without a number (defaults to 0) will be removed when dropped
Use
Side note, you might want to make your commands more strickt, not that it doesn't work anymore, but that it might not work anymore in the future