Ofgem and the price cap.

Hi all.
The way I understand it............at the beginning of this month (January 2019) Ofgem set a price cap for customers who were on their suppliers standard tariff, and pay by direct debit. I meet both of these criteria, and it's also a dual fuel tariff, which may also be a condition of the price cap.

My energy supplier is Avro energy, who have been an excellent supplier for the last 16 months, with no problems at all. When I joined them I was on a twelve month fix at £103 p/m. This ran out in October last year, when I was then moved to their standard tariff. When I looked into switching tariff, or supplier, Avro again came out cheapest, although there was a monthly increase of £25. I have looked at them again this week and the deal is still there, but now the price cap has come into effect by the regulator.
I know that Ofgem will review the price cap twice a year, and are expected to put it up in April, and potentially again in October.
Basically the maths are this:
Currently I pay 12 x £103 = £1236
If I take the new offer I'll pay 12 x £128 = £1536
The price cap is currently £1137 although likely to rise.

I don't understand why I'm told it's better for me to fix, when, if I stay on the standard tariff, I'm already paying more than the cap. What have I missed? and how does it work? Once the energy company reach the threshold of the price cap, do I pay less? Do I start to be in credit with them? Do I have to pay it and then take a refund the following month? or is the whole thing just another con. It's too hard to work out, and I can't seem to find the rules of it anywhere.