[DJ_Nik_Gnashers]

The answer given above is spot on.

If you quartz lock your turntables at 0% then the pitch is controlled by a crystal which keeps the frequency absolutely bang on 0% pitch. When you unlock this and use the pitch slider, then you are relying on the resistance of the potentiometer. Cheap turntables are pretty bad at providing you with a smooth continious curve of adjustment from the middle 0% to either the +maximum or -maximum adjustment.
Having used Gemini, Numark, stanton, Vestax & Technics turntables, the Technics are the most accurate imo. Having +or- 8% adjustment is more accurate then +or- 10% obviously, because the pitch sliders are all the same length in different turntables - then the more adjustment available, the more twitchy it is, ie it will adjust more for the same amount of movement.
If your pitch slider is dirty inside, that will make the adjustment irregular, in which case you could try cleaning them. The easiest way I have found, is to buy some really cheap/thin cotton buds (from a poundshop), bite the end so it is flat, then make a bend just before the actual cotton bud bit, spray some electrical contact cleaner on the bud (servisol super10 for example) and poke it gently into the slit where the pitch knob moves. By pressing gently the end should bend over and get into the inside edges of the pitch slider (where the tracks are that actually control the pitch circuits resistance). Move the bud up & down and use a few buds gently, if the buds are black when you slide them out of the slit, then you may notice a considerable difference in pitch accuracy afterwards.
The best way to clean the pitch slider though is to take it out of the turntable and strip it down (not reccomended unless you are extremely confident with such procedures).