I am a former military Air Traffic Controller and I agree with N..D's answer - in general. However it may be aircraft on test flights that you have seen or a trainer(2 pilot) variant on training tasks. The Red Arrows for example fly all sorts of profiles. Jet engines by their physical make-up operate best at higher altitudes and thus for longer flights that may need refuelling en-route they will get as high as is practical as soon as they are able. Hope this helps.
Theres no bigger turn-on than a jet doing its thing,showing off or not,what a thrill,and that engine,,,,phew!Hope electric dont see this thread,you'll never hear the end of it!