There are several situations that could cause your bill to change, including:
Change of Plan
Change of Billing Address
Monthly Subscriptions to Add-on Services
New Device Charged to Monthly Billing Amortization
Corrective Billing Adjustments:
* Your prorated monthly service fee (MSF) is the partial MSF covering your activation date until your statement date. This may also be charged when you avail of plan renewal, plan change, billing cycle change or reconnection from temporary disconnection.