It's pretty unlikely you have a faulty projector.
You should buy a light meter, measure the current lamp, swap the bulb, then measure the new bulb and see if there is a big difference in what the light meter says.
You did set the manual aperture setting all the way open?
A 10% to 15% variance on lamp output is fairly normal, because peoples measuring techniques are often slightly flawed + meter error + lamp variance.
However you need to measure the projector yourself, it's the only way you'll really know.
Most of the cheap meters read slightly high, and most peoples measurement techniques are slightly low, so it almost cancels the error out anyhow.
Though there is an inherent bias for some people to round up too easily, but that is a different problem.
Go grab you an lx1010B off ebay for $20...
or here is a New Extech for around $80
Watch out for CMD / frame interpolation modes enabled too, at least on older JVC's this could really mess with the lumens numbers.
Buy a light meter and check yourself, open the aperture all the way, set the projector to the brightest mode, turn off all enhancements and filters.
Take a torch mode measurement. Change the bulb and DONT touch the projectors settings, fire the projector up, take another measurement.