You won’t find a definitive explanation unless you look through the individual transactions in the sinking fund, or if you ask the Strata Manager.
Circumstances such as you describe are not ideal but are nonetheless quite common where one explanation, that may or may not be the one that’s applicable to your Plan, is where an expense arises that should legitimately be paid from the sinking fund, but due to an insufficient balance there it is paid from the administrative fund, but still booked (recorded) against the sinking fund thereby producing a negative balance.
The Strata Schemes Management Act explicitly describes what types of expenses must be paid from each fund, and in the circumstance that I’ve described here, the Owners Corporation has three (3) months to decide upon a means to fully reimburse its sinking fund, such as by raising a special levy or by increasing contributions to the administrative fund so as to produce a surplus that’s equal to the amount to be reimbursed.