Fuel injection does not "adjust itself." You need to give it more throttle
because there is much less oxygen by volume at ~6200ft. Opening the
throttle not only adds more fuel (via the injectors), but more air...then
it starts. The adjustment needs to be made because at idle, the mixture is
too rich (too much fuel, not enough O2), and the motor stalls (or won't
start). The power loss is due to an overly rich mixture. Airplanes have a
mixture control that is adjusted as they ascend or descend for exactly the
same reason.

--Mike

A modern FI system does "adjust itself" to provide altitude compensation by
use of the MAP sensor input to the computer (MEFI module). Lower barometric
pressure at high altitudes will cause a drop in manifold pressure, which
will be detected by the MAP sensor and cause the computer to adjust the
injector pulse width. Some new FI systems add a fuel pressure sensor to
provide more accurate altitude compensation. Mercury now uses their own
proprietary FI system, but in 1995 it would be a GM system.

Snowmobilers riding in the mountains have had the altitude compensation
problem for years, until FI came along and solved the problem.