Discount offered by agent was not applied to my account

I recently spoke with a representative over chat to lower my bill to match a competitor's deal at $55. I waited a billing period and the next bill showed as $120. I called on July 6th and spoke to a representative who told me that that chat never occurred and there was no promotion to match the offer.

Why was the agent able to promise a rate that was not offered?

Chat reference: 

Transcript:

Xfinity:  Hi! I’m a live agent. Can I answer any questions?

You: Starry is offering 55$ a month for the same with the model included, do you offer a service match?

You: same service with modem included*

Xfinity: Thank you for your interest in Xfinity! I hope you're doing well. Hi this is Sandra, I’ll be more than happy to assist you.

Xfinity: Ok so currently your seeing at offer for $55 with equipment

Xfinity: Can you kindly provide me you full service address to see if the offer is available for you.

You: REDACTED

Xfinity: Thank you for the information, just allow me a moment to take a look into this for you.

Xfinity: Does the address have a apartment number ?

You: REDACTED

Xfinity: Thank you.

Xfinity: Ok so i do see here that currently your you have our 500mbps.

Xfinity: Your able to upgraded to our 1000mbps and get some savings.

Xfinity: Can you kindly login to your account

You: I think I am logged in

You: Ah I'm silly. Yes. I'm logged in

Xfinity: Great so on the page that you currently are do you see a option for Customize plan ?

You: yup

Xfinity: Kindly click on it.

You: ok

You: do i go under deals or change the speed?

Xfinity: Kindly click on Change speed.

You: then customize and explore?

Xfinity: Yes

Xfinity: Then kindly click on Internet.

You: ok, I dont see 1000mbps being cheaper than what I currently have

Xfinity: Currently you have our 500mbps and your paying $120.

Xfinity: Can you kindly click on the 1000mbps.

You: ok

You: added to my cart

Xfinity: Kindly click on Next so the price that your seeing is just an estimate price.

Xfinity: Ok so now your on the modem page

You: yup

Xfinity: Kindly scroll down and check the last box.

Xfinity: Just to clarify you would not be changing your modem.

You: ok

You: then next?

Xfinity: Yes

You: now on the subscription page, click next again?

Xfinity: Yes kindly click on next.

Xfinity: Ok so now your on the review page kindly scroll down to see the price.

You: On the checkout screen, do I need to enter something else to bring the price down to the 55$ offer from starry?

You: its still showing 105

Xfinity: No you don't need to enter anything else, you can go ahead and submit your order, the new price would be reflecting on your next billing cycle.

You: The new price of 55$ correct? Not the 105 that is currently showing? I'm just going to switch services if the prices aren't comparable

Xfinity: Yes that's correct and i understand.

You: Awesome thanks!

You: Order is submitted

Xfinity: Great i do see here that your order was able to go through, so in about 5 minutes the new speed would reflect

Xfinity: The new price would reflect on your upcoming bill.

You: Thank you so much!

Xfinity: Sure your more than welcomed.

Xfinity: Thank you for visiting Xfinity today. We are looking forward to you visiting us again soon.