1/2
Double update as an apology :/
Ichigo had called Toushiro by his given name right from the start. Technically it was a matter of respect, or disrespect in this case; he thought that Toushiro was a kid who wasn't much older than his little sisters. There was no need to be formal with someone so much younger than him and whom he barely knew. So just Toushiro it was and stayed. Even when he found out that he wasn't some shinigami's kid who had gotten lost, but an actual captain, it just didn't feel right to call him Captain or Hitsugaya. He was short, he looked young, and everything about him screamed child. Granted Ichigo knew that he was a captain for a reason, but he felt that there was something wrong with putting a kid in such a position. He must have hated it; he always looked so angry and tired and was always so curt.
Then he actually met Toushiro.
The captain was young yes, but not nearly as young as Ichigo had made him out to be. And he was proud that he was a captain. All those annoyed reminders had been sincere and not a drilled formality. But he was just still Toushiro. They weren't particularly close, but it somehow felt wrong to call him by his family name, as so many others did. But honestly, his given name just slipped out, even when Ichigo tried to be a bit more polite. Ichigo never gave it a second thought. Until he thought back on how they met. He surmised that there was no one who looked to be Toushiro's age around the Seireitei. The people who were closest in age were Rukia, Renji, and their group, but they seemed ages older than him and he also had an older sister their age. The next was Yachiru and it was obvious why they weren't friends. Then there was the fact that he was a captain and had to work with adults who were centuries older than him on a daily basis. He didn't need any more formalities; he needed a friend.
Ichigo made it his goal to be his friend.
He succeeded after some years, when Toushiro stopped insisting he called him Captain Hitsugaya. He wasn't sure if Toushiro had just given up on correcting him or if it didn't matter to him any more, but he wasn't going to question it. He actually hadn't realised that Toushiro didn't correct him that time. He had more or less tuned out the automatic 'it's Captain Hitsugaya' so had gone on for a few minutes before he noticed. He paused, then tilted his head and smiled. Toushiro knew exactly why he was smiling and when Ichigo asked him about it he blushed slightly, saying that if they were friends then there was no need for such formalities. Though he didn't start calling Ichigo by his first name for a while longer. (Right before the started to date, to be precise.)
This one is a bit short because it's more of a character study than a story. It was fun to come up with a reason for that particular inconsistency.
Thanks to ShiroHalla for the donation :)