How am I wrong with this? (Bill paying issue)

Discussing anything with my wife is like arguing with a brick wall. She only sees it her way and her way is the only way it should be.

We have a number of utilities in our house and like with any household there are various log-ins online to pay the bills. Some accounts were set up in her name while others are in my name. The bills that are in her name she's given me the log-in and password to access that account. Same with me. The bills that are in my name I've given her the log-in and password to access that as well. And we're talking about bills like power, water, internet, cable, cell phone, etc. Your basic bills. A few months ago we decided to change our cell phone and internet accounts to another carrier to save us some money. Both those accounts were in my name and my wife had access to them. With our Sprint account, for some reason she wanted her own log-in even though the account was a family plan. So Sprint allowed her to set up her own log-in to access the same bill. When we went to AT&T for cell and internet apparently AT&T doesn't allows multiple accounts for the same bill so she could never gain access to the cell bill. Remind you, I already have a log-in and password to the cell account and I use the same password for all my accounts.

This morning my wife sends me a text to say what bills are due to be paid this week and when I saw the internet I thought it was a bit high so I logged into the account and noticed what my wife says we need to pay is much higher than the actual bill. We typically split the bills in half so this is the text I sent her.

Me: "Where are you getting $58.49 each for internet? Our current bill is only $98.77, $49.36 each."

Her: "Oh ok cool. I was going by the last bill we paid. You told me $115 and some change so we paid $58 and some change each last month. So is it going to fluctuate or is $98.77 the standard each month going forward?"

Me: You can't do that. The bill fluctuates month to month so check the bill before sending out the text. The way you're doing it we're paying more than what we probably should."

Her: "Remember, I don't have access to the internet bill so you can tell me anything lol but don't do that. See if you can get me access please."

The thing is this. The account is already set up but when she went to log in she either typed my email wrong or the password wrong so she couldn't get in. What did she do? Instead of saying, "hey, can you log into the internet account to see what's due this month?" No, she just made the assumption that since we paid $115+ last month it's probably $115+ this month so let's just divide that in half and each pay that amount. Again I told her that you can't make an assumption when paying bills. So now she's saying I'm calling her dumb or stupid for doing that and she feels that the way she did it is right and fine for her. Internet, water, cell phones, power, etc are not like a car or loan payment. Again, they fluctuate based on usage not based on a set amount each month. Your auto loan company isn't going to send you a bill based on how much you drove your car this past month. But I'm the wrong who is wrong because I told my wife you can't pay bills the way she has it in her head.

How am I wrong here? I guess I'm wrong for telling a woman she can't do something a certain way.

Does anyone else do it the way she does? You can't access the online account or you get a bill and just decide not to open it but just assume since you paid a certain amount last month you're going to pay that same amount this month. And when you can't access the account online do you not just pick up your phone and call to ask or in this situation as someone who has access to the account to say "can you log in and see what we owe?"