A reply suggests that the projector "has no idea whether or not you put in a new lamp" which is not true. These Metal Halide lamps have electrodes and an arc, and this technology definitely causes the ballast to have to drive higher volts to sustain the arc. From Wikipedia Metal Halide article:
"End of life behaviour
At the end of life, metal-halide lamps exhibit a phenomenon known as cycling. These lamps can be started at a relatively low voltage but as they heat up during operation, the internal gas pressure within the arc tube rises and more and more voltage is required to maintain the arc discharge. As a lamp gets older, the maintaining voltage for the arc eventually rises to exceed the voltage provided by the electrical ballast. As the lamp heats to this point, the arc fails and the lamp goes out. Eventually, with the arc extinguished, the lamp cools down again, the gas pressure in the arc tube is reduced, and the ballast once again causes the arc to strike. This causes the lamp to glow for a while and then goes out, repeatedly. In rare occurrences the lamp explodes at the end of its useful life.
Modern electronic ballast designs detect cycling and give up attempting to start the lamp after a few cycles. If power is removed and reapplied, the ballast will make a new series of startup attempts."
The projector, for safety reasons, needs to monitor the voltage and stop driving the lamp at some point.
Why does a lamp not give full life? My guess is that it is very dependent on the manufacturing process, control of tolerances, purity of materials, etc. and the 2000 hrs is the design and manufacturing goal, and the manufacturing plant has no way to functionally test each unit to be certain it will last that long. The assurance must be through process control... something that has to be re-established at every factory and changing supply chain conditions.