Why do the inlet cones on an SR-71 retract at higher speeds while the one on a MiG-21 extends at higher speeds? As shown by this illustration in the SR-71 flight manual, its inlet cones (spikes) retract as its speed increases:

 (source)

Meanwhile, the Wikipedia page for the MiG-21 describes the opposite behavior for its intake cone:


  On early model MiG-21s, the cone has three positions. For speeds up to Mach 1.5 the cone is fully retracted to the maximum aft position. For speeds between Mach 1.5 and Mach 1.9 the cone moves to the middle position. For speeds higher than Mach 1.9 the cone moves to the maximum forward position.


No source is provided for this part of the Wikipedia article. If the info about the MiG-21 is accurate, why does its intake cone behave differently than the SR-71's? Do the two aircraft's intake cones serve different aerodynamic purposes?
This is only a guess, so please take this with a grain of salt.

An important characteristic of supersonic engine inlets is the total pressure loss (or also sometimes pressure recovery). The total pressure loss over a normal shockwave is a lot higher than if you would slow down the air to the same speed through several oblique shock waves.

If you want to understand why the sr-71 spike retracts I recommend you watch this video. The sr-71 had a quite complex inlet geometry to ensure that the air was slowed down over many oblique shocks.
sr-71 engine inlet 

On jet engines that don’t have such a complex inlet geometry as the sr-71, the best pressure recovery is mostly achieved by getting the primary oblique shockwave from the spike or inlet ramp, to hit the leading edge of the engine inlet, where it gets reflected as another shockwave.
 

As speed increases the primary oblique shockwave that gets formed on the spike gets shallower meaning that if you wanted to keep the shockwave of the mig-21 spike at the leading edge of the engine inlet, you need the spike to move forward as speed increases.