The number format of a spreadsheet cell will change back to the format applied to an entire column when the cell is formatted as General and data in the cell is changed.
To replicate this issue:
1. Select an entire column by clicking in the column header.
2. Apply the currency number format or another format other than General.
3. Type some numbers into two of the cells.
4. Select one of the cells and change the number format to General.
5. Tab out of the cell to see that the number displayed returns to just the data you entered.
6. Select the cell again, click in the entry bar, and add another digit or otherwise change the value.
7. Tab out of the cell and you will see that the number format returns to the column format.