The conditions are not the same when you're alone.  Pro basketball players can easily make shots in practice but game time it's harder as there is more demand and extra variables in play.    With speaking you're at ease, breathing well, not tensing, you control the timing of when you want to speak, etc. In person you feel nerves, you're tensing, you don't always have control of the tempo and timing of when you want to speak, etc.  It's totally different.  Just like reading out loud or singing.  They are different.