Since this rumor won't step I went trough the trouble of checking all files that are relevant and would like to show the result.
There are three possibilites how an item can drop:
1. From a specific monster trough monster files.
Majority of items from normal monsters drop like this. Each monster can drop 6 different items.
2. From a specific monster trough script files.
Minority of items from monsters drop like this. This is often used for special situations like missions or raid bosses.
3. From every monster on a map trough map files.
This is used for maps like dungeons and premium maps (Ancient Dungeons, Sacred Chaos Requiem, etc.)
Item "Magical Sphere" falls into this category.
Item has ClassID="21620", players can look this up file "datatable_item_etc.xml" if they want to. These information are available on the game client.
ClassID="21620" can be found on file "datatable_map.xml" for line 194, 195,196, 198 and 200. Those lines are the Skeleton Dungeon and Ancient Skeleton Dungeon.
Example: DropItem4ID="21620" DropItem4Freq="1000"
This means the chance of the item drop at 1x droprate is 1/1000, so 0,1%.
Due to our 2x droprate the chance is 1/500, which is 0,5%.
This is the highest droprate, for other maps it is lower. These values are official, players can compare them with files from other servers.
Take into account this droprate is without the penalty that might be applied when players use high level characters.
For someone with Level 140 (High Master Level 10) characters the droprate will be penalized down to ~0,05%.
Combine that with the possibility that pet just misses Magical Sphere due to high droprate of other items, the amount of Magical Sphere per hour might be bad.
As suggested by RocGuardian, the way to go is use Veteran Level 10 characters.