I have a dwarven armour and it originally started off with 82 armour but when I loaded up my last save I noticed it had gone down to 66. Any ideas on why this would happen? TIA

Check to see if you currently have any diseases. Maybe you have something that lowers your heavy armor skill.

You probably have one the Well Fitted perk that gives +25% armor when you're wearing all heavy armor, and are not currently wearing all 4 pieces (head, chest, gloves, boots).

That's almost certain to be the case, since the deficit is almost exactly 25%. If a given piece's armor rating is 100 when you examine it or wear it alone, then you put on all heavy armor and the overall sum jumps.

Example:

I put on a helm with a rating of 30. My armor goes to 30.

I put on a cuirass with a rating of 150. My armor goes to 180.

I put on gauntlets with a rating of 30. My armor goes to 210.

Then I put on boots with a rating of 50. When I'm examining them, it says I will get a 50-point boost., But I put them on, and my armor goes to 325.

That's Well-Fitted. And I it that way!