The only way that can happen is if you gain more then 1 level up at a time but since she starts out at level 8 the only way for her to get over 100 SP is if she hits like level 12 to 15 in 1 battle.
Its not just limited to Celine if a low level party member never gained a single level up until the 13 floor bonus dungeon is accessible a single fight would cause her or him to go from like level 8 to level 20+ in 1 fight and gain the max 999 SP all at once.
Did you use any item creation with her recently? Using those can unlock talents, which is +100 SP each. --- "Why do ppl say Steam/PC has DRM? The only thing steam does is prevent piracy, and there's nothing wrong with that." - Oil_Rope_Bombs