which are explained. he has been ordered to serve Lucy any way possible to atone for him abandoning his previous summoner. But he is still team lucy (or at least one of her summons), even though eh once belonged to FT
No it isnt explained, Natsu is serving Makarov because he is a member of Fairy Tail, and Makarov is the master of that guild, but that doesnt mean Makarov can claim credit for anything Natsu does, especaly not when its something he does on his own initiative.

And in the same way, Leo might be a part of Lucy's team, but that doesnt mean she deserve credit for anything he does on his own.
Now if she actualy summon him it would be another matter, as she had then done something active towards whatever Leo might accomplish.