I see you made a duplicate post soon afterwards correcting it to say you're on a $85 plan and the bill is around $100 and you want to know why.
As this is a public forum, we don't have visibility of customer accounts.
Your bill should have a pretty clear explanation on it as to why it is higher. If it's $15 the first thing that comes to mind is could be a late payment fee? It would say this on the bill.
Other reasons include you've exceeded your plan's inclusions, used plan exclusions, other fees, etc.
If you still aren't sure why, please contact Optus directly for further assistance.
Did/do you pay the bill late?
This is an online forum, we don't have access to your account here. Please refer to the above response on how to contact Optus if you'd like to dispute the account charge.