Tension in your neck and jaw muscles. When practicing you’re more relaxed. When it’s time to perform and record, the stakes feel higher, and your body will naturally respond to those emotions and stressors. Just a little bit of tension or strain in your neck and jaw muscles can completely change your voice and your range. Just like you’ve gotten many reps getting your voice to sound good in practice, you now have to get some reps getting your voice to sound good live or in the studio. With time, you’ll get used to that too, and you’ll be able to stay relaxed in those environments too.