you found it easier to play tilted because you have created a crooked enbrochure and when you play it parallel, it doesn't produse as clear a sound because the enbrochure is crooked so try getting in front of the mirror and practicing!I actually find it easier to play with my flute tilted slightly. I get a better sound. But then again I could just be wierd!
Also, sometimes in band, people play crooked either because they dont know and follow others who also either forgot or had no clue about it. If they did know, they might tilt their flute so they don't hit someone beside them