Although Tudor uses ETA base movements, Rolex does some modifications to the base movements it receives from ETA before putting it into a Tudor.
Some of the modifications include, different mainspring (same as those used in Rolexes); modified balance bridge, instead of the standard ETA balance bridge, they replace it with a 'butterfly' balance bridge (support on both sides instead of one side only); plus they change the balance complete to those used in Rolex (Parachrom, hence the increased accuracy. AFAIK, in newer Tudor movements, the shock protection jewels are also changed to the new in-house Rolex developed system.
In essence, although the base movement is ETA, its more to a Rolex in technicality, as the important parts are replaced with those which are only found in a Rolex.