I'm no river plate expert, but top speed depends on Sea State, the heavier a ship the more it can keep up its top speed in heavy seas.
Fuel. I suppose the british burned a lot in chasing Graf Spee and during the battle, while the fuel consumtion of the Graf Spee didn't change that much.
I'm not sure Graf Spee made 28 knots in 1939.
Real top speed for the Deutschland Class was something between 26 and 28 mostly, but as I said, they could keep that up for weeks.
A nominal top speed is something not really as fixed as technical data might suggest.
As both sides fired off a lot of ammunition, the ships were considerably lighter than a standard combat load.
On the other hand, both the british and germans had spend quite some time at sea (the british even more) and the hulls must've been fouled up quite a bit.
Also, the british did not want to catch up with Langsdorff. Exeter was by all means out of action and the two Leanders were not able to fight it out with a pocket battleship.
They were shadowing Langsdorff, as light cruisers are supposed to do.
The same situation (without a battle) unfolded with Admiral Scheer in 1940.
Kapt. Krancke more or less duped the whole Royal Navy in the indian Ocean and South Atlantic, and succeeded to escape a far larger force than Langsdorff had against him.
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