I think expenses was probably added as a category so you could flag things that you’d get reimbursed from your employer but others might just use it to show up certain things (e.g. separate travel to work from leisure travel) and will still want expenses included in the overall total.
I suggest the following:
An option to excluded expenses from the overall expenses (for those who get these paid by their employer)
Still allow a target for expenses to be set even if the user wants it excluded from the main total (this will be useful if the company has an expense limit)