"I accepted Lù Yue'an's gift, and promised to help her and imitate Lu Ning's handwriting…"
Jiang Yi liked to write. Out of curiosity and interest, she would eventually learn to write in the handwriting of many people.
Over time, no matter whose handwriting it was she could imitate it very well. Sometimes, nobody could discern who had actually written it.
She did not mention this explicitly to anyone, but she often practiced calligraphy and her style would change all the time when she wrote. If someone paid attention, then they could tell with a few glances without needing her to elaborate.
In her first year of high school, Lu Ning had already noticed this.
When she realized that the handwriting was the same as hers, Jiang Yi came to her mind first.