[_Kharbon_]

Yes, I suppose there is a pattern to drops, caused by the difference in yours and the mobs level. However, I think the drop chance from all same-level creatures would be the same. Ie. it shouldn't matter which mob you drop-grind on, as long as it has the same level.
Dropped items are also random, but I tend to get loads of mage staffs, more then other items. Perhaps, there is some variable in gem/armour/weapon drop chance.

To get highest chance of items, I recommend grinding on challenging/normal mobs, as with some experience you can kill them the fastest and therefore maintain higher item/time ratio, than while killing harder mobs.

As Ichigo wrote, the chances of a same-class drop, with good stats is quite low. You will obtain best items by trading with other players, especially later in the game. There is not much point to look for good gear under level 50. Therefore my advice is, grind with whatever you get from drops/quests, until you have level 50, then start looking for better gear.

PS. Also note, that drops are close to the mobs own level.